HOMOLOGIES OF THE ALISPHENOID 
AND PETROMASTOID BONES IN 
VERTEBRATES 
BY 
HENRY C. CHAPMAN, M. D., 
•<«WSS«rtKa5**5i'' 
Professor of Institutes of Medicine and Medical Jurisprudence, 
Jefferson Medical College. 
From the Proceedings of the Academy of Natural 
Sciences of Philadelphia, 
January, 1894. 32 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
HOMOLOGIES OF THE ALISPHENOID AND PETROMASTOID BONES IN 
VERTEBRATES. 
HENRY C. CHAPMAN, M. D. 
To those unfamiliar with modern views upon the morphology of 
the skull it may not be known that any difference of opinion still pre- 
vails among anatomists as to what bones in the skull of the lower ver- 
tebrates should be regarded as homologues or representatives of the 
great wing of the sphenoid and petromastoid in the skull of Man and 
other mammalia. Considering the numerous and important palaeon- 
tological and embryological researches made in recent years, and in 
view of the fact that the remains of extinct animals and the develop- 
ment and structure of existing ones have been described so elabor- 
ately, it might naturally be supposed that the question of the special 
homologies of the bones of the skull would have long since been de- 
finitely settled. 
So far, however, from this being the case, as a matter of fact anato- 
mists, in certain instances at least, on the one hand designate the 
same bone by different names and on the other, different bones 
by the same name. The confusion of ideas thus engendered by ob- 
scure nomenclature makes it often very difficult to understand what 
particular bone is referred to, especially when the description relates 
to the cranial bones of some extinct animal. Apart from this con- 
sideration it is obvious that unless the special homologies of the bones 
of the skull of different vertebrates are determined, and identical 
bones be designated by the same name, it will be impossible to assign 
to an animal its proper position in the order of Nature or determine 
its phylogeny in the light of evolution, especially if the animal be 
an extinct one and its natural affinities obscure. The hope of dis- 
sipating some of the confusion prevailing as to the nomenclature of 
certain of the bones in the skull of Man and other vertebrates prompts 
the author to submit a resume of some well-known osteological facts 
with, however, an interpretation of the same not hitherto presented, at 
least as far as known to him. Even if the latter be not accepted, 
the determination of the synonyms of the bones in question may, at 
least, not prove superfluous hereafter to co-workers in this field 
of research. 1894.] 
NATURAL SCIENCES OF PHILADELPHIA. 
33 
The prevailing view among anatomists at the present day appears 
to be that the petromastoid portion of the temporal bone is developed 
in Man from three distinct centres of ossification, named respectively 
prootic, opisthotic and epiotic; that the prootic and opisthotic coalesce 
to form the pars petrosa, the epiotic giving rise to the specially mas- 
toid part of the pars mastoidea, and later joining the other two. 
According to this view the homologues of these three centres of 
ossification of the petromastoid of Man exist in the skull of the 
lower vertebrata either as distinct bones or more or less coalesced with 
each other or with adjacent bones. The bone formerly considered as 
the homologue of the great wing of the sphenoid or the alisphenoid, 
Figs. 1, 2, a is regarded 
as the prootic or the homo- 
logue of the upper part of 
the human pars petrosa. 
The alisphenoid proper, t, is 
often but little developed 
or even absent in the skull 
of the lower vertebrates ; 
the bone formerly named 
occipital externe or par- 
occipital is regarded as 
the homologue of the opisthotic or epiotic, while the latter is 
often represented by a small ossification coalesced with the supra oc- 
cipital. 
So far as known to the author Kerckringius is usually regarded as 
having been the first anatomist to describe 
the petromastoid portion of the temporal 
bone as developed from three centres of ossi- 
fication. Unless the author has misunder- 
stood the meaning of the text, however, that 
which Kerckringius refers to as being devel- 
oped from three centres of ossification is not 
the whole petromastoid but only that part of 
the pars petrosa which constitutes the mam- 
millary process or pars mastoidea, the pars 
petrosa proper having been more or less de- 
veloped before and independent of the three 
centres. 
Fig. 1. Side view of human skull. 
Fig. 2. Interior view 
of human skull. 34 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
In the fifth chapter of his Osteogenin Foetorum,1 published in 
1690, Kerckringius observes : " Ossis petrosi pars ilia, quae pro- 
cessuni mammilluarem constituit, tenia de novo acquisivit ossicula; 
unuin pyriforma, acutiore, sui parte squamoso annectitur; alterum 
scutum ovale referens, magnitudine, priori vix cedens, media cartila- 
gine ab co separator; uti &c., tertium ab utroque, quam vis hoc 
magnitudine neutri sit aequiparandum, vix aciculae majoris caput 
adaequans sunt autem eositu online collocata, quern tabula foetus v, 
mensium, usurpata oculis facilius ad mentem quam verba transmit- 
tet. . . . Constat ergo os temporum hoc quinto mense sex dis- 
tinctis ossiculis; osse videlicet, squamoso, anulo, osse internam 
civitatem efformante &c., tribus notabilibus quae hoc mense ex orta 
esse diximus. Sexto mense pyriforme, &c. ovale scutiforme coalue- 
runtinunum; tertium non nihil auctum est magnitudine. 
Septimo mense jam tertium illud ossiculum duobus mense superiore 
inter se coalitis accessit." 
That these three centres of ossification, viz., the "pyriforma," 
"scutum ovale" and "tertium" do not correspond exactly to the 
prootic, epiotic and opisthotic centres in the sense in which these 
terms are used by modern anatomists, appears from the text of the 
latter part of the passage quoted, beginning " constat ergo," the tem- 
poral bone being said to consist at the fifth month of six distinct os- 
sicles, which would be impossible if two of the "tribus notabilibus" 
coalesced to form the "osse internam cavitatem efformante," which, 
however, as we shall see hereafter, they do a little later. That by 
the "osse internam civitatem efformante " is meant that part of the 
pars petrosa developed before and independent of the appearance of 
the three distinct ossicles becomes evident from the description of the 
state of the development of the temporal bone at the previous or 
fourth month, Kerckringius stating that the bone at the end of that 
month consists of three ossicles: "annulo, " " osse squamoso " and 
"illo jam commemorate," the latter being the "osse internam" or 
that part of the pars petrosa developed at that period. ' ' Quarto 
mense minim visu, quam cito & quanto perfectione os squamosum 
magnam partem factum sit osseum. Os petrosum jam rubicunda 
cartilagine signa vitcavitatis suae for mam organorum audituscapacem, 
nihil tamen adhuc praesefert osseum, praeterquam imam in longitudi- 
1 Theodori Kerckringii, Osteogenia Foetuum. Bibliotheca Anatomica, T. 
II, Genevae, 1685. 1894.] 
NATURAL, SCIENCES OF PHILADELPHIA. 
35 
nem protensam crassiusculam & inaequalem lineam, annulo seu 
circulo, antea nominate, subjectam &c. paulo longius protensam. 
Os itaque temporum hoc mense tribus constat ossiculis, aunulo 
scilicet,2 osse squamoso et illo jam commemorate. 
Further, according to Kerckringius the "pyriforma" (prootic) 
coalesces at the sixth month with the "scutum ovale" (epiotic) to 
form one bone, the latter being joined at the seventh month by the 
"tertium illud ossiculum" (opisthotic), whereas, as we shall show, 
it is the opisthotic and not the epiotic that combines with the prootic 
to form the pars petrosa. As to what Kerckringius meant by his 
" tria petrosi ossis distincta ossicula," we will try to make clear in 
our description of the manner in which the petromastoid develops 
in Man. 
The work "De Aure Humana" by Cassebohm appeared in 1734- 
1735. The description of the development of the ear given by 
this author, while agreeing in the main with that of Kerckringius, 
differs from it in some respects. Cassebohm recognized, however, 
with Kerckringius, the fact of the pars petrosa being more or less os- 
sified before the appearance of the " tria ossicula," as shown by 
numerous passages in his text as well as by his excellent figures.3 
In the judgment of the author, Meckel, rather than Kerckringius 
or Cassebohm, was the first anatomist to describe the petromastoid of 
the temporal bone as arising from three distinct centres of ossifica- 
tion. According to this author4 the first part of the labyrinth to os- 
sify is that immediately surrounding the fenestra rotunda, whence 
2 Kerckringius, loc. cit. 
3 Tractatus Quatuor Anatomici de Aure Humana. Halae Magdeburgicae, 
1734 p. 19 Kerckringii relatio de osse temporum in foetu trium mensium, 
cum mea observatione de foetu totidem convenit excepto. . . . Idem ille 
porro in foetu quinque mensium, partem petrosam, circa processum mastoideum 
tria ossicula habere refert, quorum due mense sexto coaulere: verum in his 
mensibus. in loco indicate nulla ossa peculiaria sed cartilaginem vidi Tab. 3, Fig. 
22, lit. K." 
Tractatus Quatuor &c., p. 45:-Antequam tympani membranam removebam, 
cochleae tuber membranae hujus partem superiorem & mediam tangebat & 
quam vis cartilaginem esset, foraminis tamen rotundi (quod ad cochleam & 
ducit & pertinet) margo erat osseus partem que annuli posteriorem attingebat." 
. . . . Cochleae que tuber ex fusco quern habebat, colorem nigrum acquisi- 
visset, tunc foraminis rotundi margo albus osseus que apparebat." 
Tractatus Quintus Anatomicus. Halae Magdeburgicae, 1735, p. 15:-" In foetu 
trium mensium cochleam, circa foramen rotundum osseam, reliquam vero coch- 
leae partem cartilagineam. In foetu quatuor mensium cochleam vidi osseam, 
excepta lamina spirali, quae cartilaginea erat; hanc in foetu quinque mensium 
osseam demum conspexi." 
4 Handbuch der Menschlichen Anatomie, Band 4. Halle & Berlin, 1820, p. 49. 36 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
the spreading ossification produces the floor of the labyrinth. The 
second centre of ossification arising at the extreme end of the su- 
perior vertical canal, gives rise to the internal auditory meatus and 
the floor of the cochlea. The third centre of ossification begins as a 
scale upon the middle of the internal vertical semi-circular canal. 
It is to be inferred that Meckel supposed the mastoid is developed 
from the latter centre, for though he does not say so in the passage 
just referred to, as he had some years previously5 expressed the view 
that the mastoid is developed from a special centre distinct from that 
forming the pars petrosa, and as the latter is formed according to 
Meckel from the first two centres, the only centre remaining to form 
the mastoid is the third centre, that beginning as a scale upon the in- 
ternal vertical semi-circular canal. 
In his well known work on the temporal bone Hallmann also ex- 
presses the view that the mastoid portion is developed from a special 
centre of ossification, offering as a proof thereof the fact of his being 
able to scrape off, in certain specimens of the dried skull, the third 
oval centre of ossification without injuring the semi-circular canal.6 
In 1848 Rathke, in describing the development of the turtle, re- 
garded the two bones, the rocher7 and occipital externe8 of Cuvier, 
which, according to this author, form the labyrinth, as corresponding 
when taken together to the pars petrosa of the temporal bone of 
higher vertebrates:-" Die Knorpel Kapsel, welche sich auch bein 
den Schildkrceten um den hautigen Theil des Ohr labyrinthes bildet 
und anfangs eine sehr einfache Form und nursehr dfinne Wandung 
hat, bleibt in ihrer gegen das Gehirn gekehrten Halfte bei mehreren 
Schildkrceten (namentlich in den Gattungen Emys und Chelonia), 
wenn nicht gar bei alien, zeitlebens knorplig. Hire iiussere und 
groessere Halfte hingegen verknochert allmaehlig, so jedoch, dass 
in ihr zwei Knochenstiicke einstehen, von denen ein jedes einen Theil 
des hautigen Ohrlabyrinthes, namentlich Theile der halbzirkel 
formigen Kanale einschliesst und von denen nur das eine mit dem 
Namen des Felsenbeins belegt, das andere von Cuvier nicht recht 
5 Archiv, Band 1, 1815, p. 636. 
6 Die vergleichende Osteologie des Schlafenbe.ins, Hannover, 1837, p. 3:- 
" Dieser Knochen kern zeigt sich am trocknen skelet als ein ovales Knopfchen, 
das ich leicht abkratzen konnte, ohne die kanale zu verletzen, zum zeichen 
dass dieser Theil als ein besonderes stuck entsteht." 
7 Alisphenoid, Prootic. 
8 Paroccipital, Opistbotic. 1894.] 
NATURAL SCIENCES OF PHILADELPHIA. 
37 
passend Os occipitale externum genannt worden ist. Beide Knochen- 
stiicke nun, die zusammengenommen eigentlich nichts anderes als 
das Felsenbein der hohern Thiere vorstellen.9 
About the same time the late Professor Owen10, in referring to the 
views of Kerckringius as to there being "tria petrosa ossis distincta 
ossicula, " called attention to the primary independence of the base 
of the processus mastoideus as " a fact of much more significance 
than its brief and transitory manifestation would lead the author to 
divine." He further observed that "in the cold blooded vertebrates 
the mastoid retains with a few exceptions its primary embryonic dis- 
tinctness as an independent element of the skull." It should be 
mentioned in this connection, however, that as Professor Owen re- 
garded the bone now considered in the lower vertebrata as the 
squamosal to be the homologue of the special centre of ossification of 
the human pars mastoidea, he calledit accordingly the mastoid. 
In 1861 the view of Meckel as to the original distinctness in the 
embryo of the three parts of the petromastoid portion of the tem- 
poral bone was confirmed by Kolliker11 in his description of the de- 
velopment of that bone in Man. 
Finally, Professor Huxley in his lectures as Hunterian Professor 
at the Royal College of Surgeons of England, in 1863, and em- 
bodied substantially in his "Lectures on the Elements of Compara- 
tive Anatomy," 12 developed at length and in detail the view that the 
petromastoid portion of the temporal bone in man is developed from 
three centres as described by Meckel and Kolliker, and further that 
these three centres are represented in the lower vertebrata as more 
or less distinct bones as held by Rathke and Owen. 
The two bones described by Rathke as forming in the turtle the 
"felsenbein" were named by Prof. Huxley the prootic13 and opistho- 
ticu; that regarded by Owen as the mastoid was considered by Prof. 
Huxley to be the squamosal15, and the "specially mastoid" part of 
the "pars mastoidea" the "scutum ovale referens" of Kerckringius 
was named by Prof. Huxley the epiotic16. 
9 Ueber die Entwickelung der Schildkroten. Braunschweig, 1848, p. 52. 
10 On the Archetype and Homologies of the Vertebrate Skeleton. London, 
1848, p. 29. 
11 Entwicklungsgeschichte des Menschen und der Hoheren Thiere. Leipzig, 
p. 320. 
12 London, 1864. 
13 Op. cit. p. 222. 
14 Op. cit. p. 222. 
15 Op. cit. p. 230. 
16 Op. cit. p. 155. 38 
PROECEDINGS OF THE ACADEMY OF 
[1894. 
The view of the morphology and terminology of the periotic cap- 
sule and adjacent bones as sustained by Prof. Huxley and most other 
anatomists of the present day is based upon two assumptions: 
1st. That the petro-mastoid portion of the temporal bone in Man 
develops from three distinct centers of ossification named respectively 
prootic, opisthotic and epiotic and represented in the skull of the 
the lower vertebrata by three bones bearing respectively the same 
three names and more or less coalesced with each other or adjacent 
bones. 
2d. That the bone lying in front of the exit of the inferior max- 
illiary division of the fifth nerve should be regarded in the lower 
vertebrata as the homologue of the great wing of the sphenoid in Man 
and named the alispheuoid, and the bone lying behind such exit as 
homologous with the upper part of the pars petrosa of Man and 
named the prootic. 
The conclusion that follows if these two assumptions be admitted, 
will be that the alispheuoid or homologue of the great wing of the 
sphenoid is often but little developed or may be even absent in the 
skull of the lower vertebrata, its place being supplied by the prootic 
bone or the homologue of the upper part of the pars petrosa of man. 
Notwithstanding the high authority of Meckel, Kolliker, Huxley 
and others, among whom may be mentioned the late W. Kitchen 
Parker, universally conceded while living to be the highest authority 
on all questions pertaining to the development and morphology of 
the skull, the three-fold development of the petro-mastoid portion of 
the temporal bone has been denied by anatomists and notably by the 
late Dr. Joseph Leidy.17 
The author having had occasion recently to study the development 
of the temporal bone in Man has satisfied himself, at least, that the 
mastoid portion of the petrosal is not developed from a special centre 
of ossification but from the petrosal and squamosal portions of the 
temporal as described by Leidy,18 and that there is no homologue, 
therefore, in the skull of Man, of the bone described as the epiotic 
in that of the lower vertebrata. Further, while there is no doubt 
that the petrosal part of the temporal in Man is developed, as we shall 
17 Science, Vol. 1. No. 18. June 8, 1883, p. 507. 
18 Op. cit. p. 507, Human Anatomy, 1889, p. 116. 1894.] 
NATURAL SCIENCES OF PHILADELPHIA. 
39 
see presently, from two distinct centres of ossification, these two 
centres, in the judgment of the author are not so exactly repre- 
sented in the skull of the lower vertebrata that the names pro- 
otic and opisthotic can be given them. 
At a period about the middle of the fifth month of intra-uterine 
life the temporal bone exhibits, according to the dissections of the 
author, three distinct ossifications: the squamosal, the tympanic and 
a ring surrounding the foramen rotundum of the otherwise cartila- 
ginous labyrinth. A little later a second centre of ossification makes 
its appearance at the extreme end of the superior vertical canal. 
From these two centres of ossification, which we will name the upper 
and lower otic, as indicating their situation in Man, there arise, due 
to progressive ossification extending through the labyrinth, the pars 
petrosa and two-thirds of the pars mastoidea of the temporal, the 
remaining third of the latter being developed from the squamosal. 
From the lower otic ring-like ossific centre surrounding the foramen 
rotundum arise all that part of the pars petrosa seen beneath the 
cranium, viz.: the lower part of the cochlea, the promonotory, and 
lower part of the fenestra ovalis, the fenestra rotunda, the lower 
arm of the posterior semicircular canal, the lower part of the facial 
canal, jugular fossa, the carotid canal and the floor of the tympan- 
num. From the upper otic ossific centre, (that appearing on the 
superior vertical canal) arise all that part of the pars petrosa seen 
within the cranium except that entering into the formation of the 
jugular fossa, viz.: the upper part of the cochlea includingthe cupola 
and the base, the internal auditory meatus, the upper part of the 
fenestra ovalis, the upper arm of the posterior and the superior and 
vertical semicircular canals, the upper part of the facial canal, the 
tegmen tympani. 
The various structures just described as respectively produced from 
the two ossific centres having coalesced about the eighth month19 or even 
earlier, to form the osseous labyrinth, there is developed as a continuous 
outgrowth of the latter the pyramidal and mastoid portions of the 
petrosal, the mastoid being formed more especially by outgrowths 
from the posterior and external semicircular canals. The outgrowth 
from the posterior semicircular canal appears as an "elliptical islet"20 
19 It should be mentioned in this connection that it is often impossible to de- 
termine exactly the age of a foetus, since foetuses of the same age vary as re- 
gards length, weight and development. 
20 " Epiotic," Huxley, Op. cit. p. 155. 40 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
(Fig. 3, 4, e), in the cartilage situated between the squamosal, parie- 
tal and occipital bones and consti- 
tuting, t herefore, part of the wall of 
the cranium. The elliptical islet 
having been developed, a second 
"quadrate " islet (Fig. 3, 4, q) now 
appears in the cartilage and more 
particularly in the part of it lying 
between the elliptical islet and the 
squamosal. The two islets, the ellip- 
tical and quadrate, subsequently 
uniting together form the mastoid 
portion of the petrosal. If the lat- 
ter be developed in the manner just described then the " pyrifor- 
ma" and "scutum ovale" of Kerckringius would be the parts 
described by Leidy as the "elliptical" and "quadrate" 
islets, since the former, like the latter, "coalescent in unum" 
but not to the prootic and opistho- 
tic centres, since the latter are de- 
veloped and coalesce before the islets 
even appear, and for the reason 
already given that it is the pyri- 
fornia or prootic and the scutum 
ovale or epiotic that unite according 
to Kerckringius, not the prootic 
and opisthotic. 
The third ossicle of Kerckringius, however, the "vix acicuhe 
majoris caput adaequans" corresponds to that part of the lower otic 
or opisthotic that, growing outward and backward, makes its ap- 
pearance at the edge of the tympanic ring at an 
early period of intra-uterine life and which soon 
combines, as we have seen, with the upper otic or 
prootic to form the pars petrosa, the latter subse- 
quently uniting with the squamosal to form the 
mastoid portion of the temporal. 
If, however, there is no distinct mastoideus or 
epiotic centre of ossification in the temporal bone of 
Man what interpretation is there to be offered as to 
the homologies of the bones present in the fish and 
Fig. 3. Temporal bone of human 
foetus. 
Fig. 4. Temporal bone of human 
foetus. 
Fig. 5. Upper anter- 
ior portion of skull 
of cod-fish. 1894.] 
NATURAL SCIENCES OF PHILADELPHIA. 
41 
turtle and described by Cuvier 31 as mastoideus and occipital externe, 
by Owen 32 as mastoid and paroccipital, and by Huxley 23 as squa- 
mosal and epiotic in the fish and squamosal and opisthotic in 
the turtle? In reply to such query, in the judgment of the author 
the bone, No. 8, (Figs. 5, 6, 7, 8) described by Cuvier as mastoidien 
and by Owen as mastoid in the skull of the lower vertebrates, 
should be regarded as it is by Huxley and most modern 
anatomists, as the homologue of the 
squamosal (Fig. 3, s) of the temporal 
of Man. The bone No. 4, (Figs. 5, 
8, the occipital externe (perch) Cu- 
vier, the paroccipital of Owen, the 
epiotic, of Huxley in fish, opisthotic 
in turtle, is not, however, represent- 
ed as a distinct bone in the skull of 
Man but as the "eminentia aspera"24 
of the occipital bone or the "scabrous ridge extended from the 
middle of the condyle towards the roots of the mastoid process."25 
The name occipital externe, or its 
English equivalent external occipi- 
tal, may as well then be retained 
for the bone No. 4 as simply ex- 
pressing the fact that there exists in 
the skull of the lower vertebrates a 
bone lying external to the supra and ex-occipital irrespective of any 
preconceived hypotheses. The names paroccipital and epiotic should 
be discarded, as the former implies that the bone No. 4 is the para- 
pophysis of the first cranial vertebra, the latter that it is the 
homologue of the special centre of ossification of the mastoid in Man. 
It has been urged in favor of the bone No. 4 being called the epi- 
otic in the fish that it enters into the formation of the ear-chamber, 
its inner surface being excavated for the reception of part of the 
posterior and external semicircular canal. Such argument, however, 
loses all force when it is remembered that the exoccipital is similarly 
Fig. 6. Side view of skull of cod-fish. 
Fig. 7. Skull of python. 
21 Histoire Naturelie des Poissons, Tome 1, p. 236. 
22 Anatomy of Vertebrates, Vol. 1, p. 97. 
23 Op. cit. p. 174. 
24 S. T. H. Soemmering " De Corporis Humani Fabrica," T. 1,1794, p. 105. 
25 Alexander Monro. "The Anatomy of the Humane Bones," Edinburgh, 1732, 
p. 110. 
4 42 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
excavated, and that the latter bone, together 
with the alisphenoid (prootic) petrosal (opis- 
thotic) squamosal and post-frontal bones 
enter into the formation of the otocrane. It 
should be mentioned in this connection that 
the external occipital bone, No. 4, (Figs. 5 
and 8), though presenting in the turtle the 
same characters and similar relations as in 
the fish, is usually described in that animal 
and in the alligator, etc., as the opisthotic bone, 
the epiotic being supposed then to be repre- 
sented by an independent centre of ossifica- 
tion which, while distinct at an early period 
of life, later coalesces with the supra-occipi- 
tal, No. 3-f- (Fig. 9). 
Even if such be universally the case it 
would only prove that either the supra-occipital develops in reptilia 
from two centres in the embryo, or that there exists a bone (epiotic) 
in the skull of reptiles that has no homologue in that of fish or Man, 
not that the bone No. 4 is the opisthotic. Indeed, as we shall see 
presently, there is no reason to believe that an opisthotic bone that 
is the supposed homologue of the rocher or petrosal in the fish (cod) 
of the lower part of the pars petrosa of Man exists in the skull of 
the reptile (turtle) at all. 
Admitting that the petromastoid portion of the temporal bone in 
Man develops from two centres of ossification, it remains now to de- 
termine whether there exists in the skull of a cod-fish, snake, turtle 
or alligator the homologue of these two centres, two bones to which 
the names prootic and opisthotic can be appropriately given and 
which, taken together, represent therefore the pars petrosa of the 
of the temporal bone in Man. 
In considering this question let us begin by first pointing out in 
what respect the bone No. 6, (Fig. 6)- in the cod resembles and 
differs from the prootic or upper portion of the human pars 
petrosa, (Fig. 2, p). It resembles it in its inner concavity 
usually supporting the anterior part of the vestibule and the 
anterior vertical semicircular canal. It differs from it in not 
presenting a fenestra ovalis, a cochlear roof, osseous semicircular 
canals, internal auditory meatus or tegmen tympani, no such parts 
Fig. 8. Posterior view of 
skull of turtle. 1894.] 
NATURAL, SCIENCES OF PHILADELPHIA. 
43 
being present in the cod, and in not transmitting the portio dura of 
the seventh nerve, the latter also not existing in the cod-fish.26 If the 
bone No. 6 in the cod-fish be compared, however, with 'the great 
wing of the sphenoid in Man (Figs. 1, 2, a) it will be observed that it 
agrees with the latter (alisphenoid) in articulating with that part of 
the basi-cranial axis corresponding to the basi-sphenoid, and with the 
parietals in entering into the formation of the lateral wall of the 
cranium, supporting the mesencephalon, and in being notched anter- 
iorly (perforated in the carp) for the transmission of the superior and 
inferior maxillary branches of the fifth nerve. The bone No. 6 in 
the cod differs from the alisphenoid in Man, as already mentioned, in 
supporting part of the membranous labyrinth and in the notch trans- 
mitting the inferior maxillary branch of the fifth nerve being situated 
in the anterior part of the bone rather than in the posterior part as 
is the case in Man. 
In the consideration of the relative posterior of the notch v in the 
bone No. 6 in the fish and that of the foramen ovale in the alis- 
phenoid of Man, (Fig. 2, /o), the fact appears to have been entirely 
lost sight of that the notch or foramen in the fish corresponds to two 
distinct foramina in Man: the foramen rotundum, (Fig. 2, r) and 
the foramen ovale, (Fig. 2, /o) transmitting respectively the super- 
ior and inferior maxillary branches of the fifth nerve, and that the 
part of the notch in the fish, (Fig. 6, V) corresponding to the foramen 
rotundum in Man, is situated anteriorly just as is the case in Man. 
The situation of the exit of the superior maxillary nerve is therefore 
substantially the same in the bone No. 6 in the cod (and absolutely 
so in the carp) as in the alisphenoid of Man. The objection that 
might still be urged that that part of the notch corresponding to the 
foramen ovale is situated anteriorly in the fish but posteriorly in 
26 The author is familiar with the view entertained by some anatomists that 
hyomandibular branches of the fifth nerve represent in the fish the branches of 
the portio dura of the seventh nerve or facial in Man. Such an interpretation 
is, however, untenable, being based upon the assumption that the quadrate bone 
(jugal caisse hypotympanic) in the fish is the homologue of the incus in Man, the 
articulare corresponding then to the malleus. As the quadrate and malleus are, 
however, developed as ossifications of the proximal ends of Meckel's cartilage 
(mandibular arch) the quadrate must be the homologue of the malleus, not the 
incus, if it be homologous with either of the ear bones. The hyomandibular bone 
(temporal mastoidien epitympanic) in the fish is homologous with the incus, 
these bones being developed through the ossification of the proximal ends of the 
hyoid arch. It must be admitted, however, that this last view leaves still un- 
explained why the articulare in the mandibular arch of the fish and the same 
bone together with the others entering into the formation of the lower jaw of the 
alligator are not represented in Man. 44 
[1894. 
Man lias no significance, since in Man and mammals generally there is 
always a small portion of the alisphenoid behind the foramen ovale, 
amounting, indeed, in the sheep to about one-half the bone. 
The statement often made that the bone lying behind the exit of 
the fifth nerve is the homologue of the upper part of the human pars 
petrosa is simply not correct so far as concerns Man and mammals. 
In view of the facts just mentioned some anatomists have con- 
sidered the bone No. 6 in the fish as the homologue of the alisphen- 
oid in Man and have so named it. Other anatomists, on the other 
hand, impressed with the fact that the bone supports a part of the 
membranous labyrinth, have regarded it as the homologue of the 
upper part of the human pars petrosa and called it accordingly pro- 
otic. In the judgment of the author an insuperable objection to 
accepting the latter view is that it involves the inevitable but absurd 
conclusion that its homologue, or the prootic portion of the pars 
petrosa, must transmit the superior and inferior maxillary branches 
of the fifth nerve. On the other hand, it might be urged that the 
bone No. 6 can not be the homologue of the alisphenoid in Man 
since the latter never supports any part of the membranous labyrinth. 
In reply to the latter objection, though at the risk of committing a 
petitio principii, the author must say that it is just in this respect 
that the skull of the cod- 
fish differs from that of 
Man : The bone No. 6, 
in the fish, the homo- 
logue of the alisphenoid 
in Man, protects the an- 
terior part of the laby- 
rinth as is done by the 
upper part of the human 
pars petrosa, the differ- 
ence being conditioned 
by the part of the mem- 
branous labyrinth being 
relatively enormously 
developed in the fish, the osseous covering but little so, whereas in 
Man the labyrinth is but little developed while the pars petrosa is 
much so. 
If this view be correct then the bone No. 6 in the fish 
must be regarded as the homologue of the alisphenoid in Man 
PROCEEDINGS OF THE ACADEMY OF 
Fig. 9. Interior view of auditory region of alligator. 1894.] 
NATURAL SCIENCES OF PHILADELPHIA. 
45 
and the name given to it by Cuvier of grand aile or its English equiv- 
alent alisphenoid (Owen) retained.27 If the bone No. 6 in the skull of 
the python (Fig. 7), turtle (Fig. 10), alligator (Figs. 9, 11), be 
compared with that so numbered in the cod-fish (Fig. 6), it will 
be found that while it resembles the latter in articulating with the 
basisphenoid and parietal bones, entering into the formation of 
the lateral wall of the cranium, presenting a notch or foramen for 
the transmission of the superior and inferior maxillary branches 
of the fifth nerve and protecting the anterior part of the organ 
of hearing, it differs from it in transmitting the filaments of the 
portio dura and mollis of the seventh nerve and in forming the 
anterior half of the fenestra ovalis of the vestibule. 
In the latter respects, and in protecting the anterior part of the 
labyrinth, the bone No. 6 in the reptile certainly resembles 
Fig. 10. Interior view of auditory region of turtle. 
the upper or prootic part of the human pars petrosa and would 
be more appropriately named, therefore, the prootic than the bone 
No. 6 in the fish. The fact, however, of this bone in the reptile 
transmitting by notch or foramen28 the maxillary branches of the 
27 It should be mentioned in this connection that there are present in the 
skulls of certain fishes (salmon and carp) three bones which have been regarded 
as the prootic, alisphenoid and orbito-sphenoid. As the latter bone, the most 
anterior of the three, is, however, inconstant and as we regard it when present as 
an interorbital bone, not the orbito-sphenoid, its presence or absence will not 
affect the argument as stated above, the so-called prootic in the carp, for ex- 
ample, being the alisphenoid, and the alisphenoid being the orbito-sphenoid. 
28 It will be observed in the case of the python (Fig. 7) that the two foramina 
in the bone No. 6 are as well marked as the foramen rotundum and foramen 
ovale are in the alisphenoid of Mau. 46 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
fifth nerve is irreconcilable, as in the case of the fish, with the idea 
that it is the homologue of the upper prootic portion of the human 
pars petrosa. On the other hand, if the bone No. 6 in the reptile be 
regarded as the homologue of the alisphenoid in Man the difficulty 
presents itself that the former enters into the formation of the fenestra 
ovalis and transmits in the reptile the filaments of the facial and 
acoustic nerves, which the alisphenoid of Man never does, the fenestra 
rotunda and the nerves being confined to the pars petrosa. With 
the view of reconciling these difficulties the bone No. 6 in the reptile 
has been regarded by some anatomists as consisting really of two 
bones fused into one, the anterior and posterior parts being viewed 
respectively as alisphenoid and prootic bones. 
The study of the development of the reptilian skull offers some 
confirmation of this view, since as a matter of fact, according to 
Parker,29 the bone No. 6 in the snake develops from two centres, the 
anterior of which is regarded as the alisphenoid, the posterior as the 
prootic. If the latter view be accepted the result of development in 
the skull of the snake is very different from that in Man, since the 
prootic ossification, instead of combining with the opisthotic to form 
the pars petrosa, unites with the alisphenoid to form one bone. 
Further, it does not follow because the bone No. 6 develops from two 
centers of ossification that it must necessarily be regarded as consist- 
ing of two distinct bones, any more than the basi sphenoid must be 
regarded as consisting of three bones because it develops from three 
centres.30 It seems to the author that the view most reconcilable 
with the facts of development as well as those relating to the adult 
condition of the skull is to regard the bone No. 6 in the reptile as the 
homologue of the bone so numbered in the fish and of the alisphenoid 
in Man. 
In the absence of petrosa in the skull of the reptile and 
bird some other bone or bones must fulfil the functions of that bone 
in supporting and protecting the labyrinth and in transmitting the 
facial and acoustic nerves. These functions are filled in the reptile 
more or less by the bones Nos. 6, 4, 2, 3, which we regard as the 
alisphenoid, external occipital, ex-occipital and supra-occipital, or, 
29 The Morphology of the Skull, London, 1877, p. 204. 
30 One for the median basisphenoid, two for the symmetrical basitemporals, 
the homologues of the lingulae sphenoidales of Man. 1894.] 
NATURAL SCIENCES OF PHILADELPHIA. 
47 
in regard to the latter more particularly, its inferior and internal 
part (epiotic),31 No. 3 + (Fig. 9). 
There remains now for consideration the question as to how much 
of the lower portion of the human pars petrosa is represented in the 
skull of the lower vertebrata. In other words, is there any distinct 
bone in the skull of the lower vertebrata to which the name opistho- 
tic can be appropriately given ? In the skull of the cod-fish, as in 
that of the Gadidae generally, there exists, though often but little 
developed or even absent in many fishes, a large and conspicuous 
bone, No. 16, (Fig. 6) which articulates with the basi-occipital, basi- 
sphenoid, ex-occipital, par-occipital, squamosal and alisphenoid bones 
and forms the posterior lateral wall of the cranium. This bone, No. 
16, on account of supporting that part of the membranous labyrinth 
containing the otolithes has been usually regarded by anatomists (Cuv- 
ier, Owen, Huxley) as corresponding to the whole of the human pars 
petrosa or at least to some part of it, and has been accordingly named 
rocher, petrosal, opisthotic, etc. In the fish the labyrinth, however, is 
not exclusively and entirely enclosed by a special osseous covering as in 
Man. The cavity enclosing the organ of hearing is formed not only 
by the bone No. 16, but by the alisphenoid, ex-occipital, par-occipital, 
squamosal and post-frontal bones as well. It opens widely into the 
cranial cavity. It presents nothing comparable to the fenestra ovalis 
and fenestra rotunda of the pars petrosa. Such being the case it is 
impossible to determine whether the bone No. 16 in the fish repre- 
sents the whole, or only a portion and more particularly the lower or 
opisthotic portion of the human pars petrosa. The author would 
therefore prefer to call the bone No. 16 in the fish simply the rocher 
or its equivalent, the petrosal, as indicating the probability of it cor- 
responding to some part of the human pars petrosa. 
The term opisthotic is objectionable as not only implying that the 
bone No. 16 in the fish corresponds to the lower or opisthotic part of 
the human pars petrosa, for which view there is no evidence, but 
further, for the reason already given that it is the bone No. 4 in the 
fish, not the bone No. 16, that is the homologue of the external 
occipital, the so-called opisthotic in the turtle. Indeed, the bone 
No. 16 of the fish does not appear to be represented as such either in 
31 Even Parker admits that " in some forms the periotic bones do not arise 
separately, but the supra-occipital and ex-occipitals extend into the epiotic and 
opisthotic regions respectively." Op. cit. p. 349. 48 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
the reptile or in the higher vertebrata. In some respects it is a 
peculiarly ichthyic bone though not invariably present even within 
the limits of the class, as already mentioned. 
The ear-chamber in reptiles is more or less closed, internally at 
least, in the adult condition by three bones separated by a Y-shaped 
suture distinctly visible in the longitudinally divided skull (Fig. 9). 
The two lower, No. 2, 6, (Fig. 9,) of the three bones are situated 
on either side of the vertical stem of the Y-shaped suture, the third 
remaining bone No. 3 within the diverging branches of the latter. 
Externally the osseous vestibule presents a fenestra rotunda far 
(Fig. 11), situated entirely within the bone No. 2 and a fenestra ovalis, 
fno, the posterior half of which is formed by the margin of the bone just 
mentioned, the anterior half by that of the bone No. 6. The mem- 
branous labyrinth consists of a vestibule, semicircular canal and, in 
the turtle and alligator, of a rudimentary cochlea. The fenestrse are 
closed by membranes in the living animal and to that of the fenestra 
ovalis is attached a col- 
umellar-like bone which, 
on account of being con- 
nected with the mem- 
brana tympani, is re- 
garded as the homologue 
of the stapes of the hu- 
man ear. The membra- 
na tympani is attach- 
ed in turn to bone 
No. 28, (Figs. 7, 10) 
usually called the quad- 
rate and regarded as 
representing either the tympanic bone or the malleus in Man.32 
The tympanic membrane being so superficially situated, neither 
external auditory meatus nor external ear can be said to exist in 
reptiles. The nearest approach to an external ear is seen in the 
crocodilia which are provided with two cutaneous folds situated just 
Fig. 11. External view of auditory region of 
alligator. 
32 Some of the reasons that may be regarded in favor of accepting the latter 
hypothesis as the correct one have already been stated. 1894.] 
NATURAL SCIENCES OF PHILADELPHIA. 
49 
outside the membrana tympani which, when approximated, close the 
entrance thereto. In the cro- 
codilia each tympanum (Fig. 
12, T) is not only provided 
with its own lateral Eustachian 
tube proper, LE, but the two 
tympana communicate directly 
with each other superiorly by 
means of a passage traversing 
the supra occipital bone, S, 
and inferiorly by anterior 
lateral passages, C, descending 
from the floor of the tympanum 
into the anterior branch, A, of median Eustachian tube, ME, situated 
just behind the posterior nares and indirectly by posterior lateral pas- 
sages, B, given off on either side from the lateral Eustachian tubes proper, 
LE, and terminating in the posterior branch, P, of the median Eus- 
tachian tube, ME. Further passages extend from each tympanum, 
Q, through the quadrate, thence by a membranous tube into the 
articulare of the lower jaw.33 
In the turtle the most posterior of the two lower bones entering 
into the formation of the ear-chamber is a distinct bone, No. 4, (Fig. 
10), and, for reasons already given, is regarded by the author as homo- 
logous with the external occipital of the fish and is therefore similarly 
numbered and named. In the lizard, python and alligator the 
functions of the external occipital bone of the turtle, No. 4, (Fig. 10), 
are, however, filled by what appears, in the adult skull at least, to be 
a part of the ex-occipital bone, No. 2*, (Fig. 9). At an early period 
of development in the snake and possibly in all reptilia, this part of 
the ex-occipital exists as a distinct bone, notwithstanding that in later 
life it may have coalesced to such an extent with the ex-occipital 
that its original distinctness is entirely lost. If such be the case, 
which is not at all improbable, then that part of the ex-occipital in 
the alligator, No. 2*, (Fig. 9), entering into the formation of the in- 
ternal ear-chamber, should be regarded as the homologue of the ex- 
ternal occipital No. 4, (Fig. 10), in the turtle. The only essential 
Fig. 12. Diagram of tympanic canals of 
alligator. 
33 Windischmann. De Penitiori auris in Amphibiis structure, 1831. 
Owen, Phil. Trans., 1850. 
Stannius, Handbuch der Zootomie, Zweite Aufl., Zweites Buch, 1856, pp. 
58, 164. 50 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
difference in the two would then be that in the alligator the external 
occipital coalesces with the ex-occipital, whereas in the turtle it re- 
mains a distinct bone throughout life. It will be observed, how- 
ever, whether the osseous part in question be regarded as an out- 
growth of the ex-occipital or as a distinct bone coalescing with the lat- 
ter, that in neither case would the name opisthotic be appropriate, 
since this bone or part, being homologous with the external occipital, 
should be so named. 
It has already been mentioned that that part of the supra-occipital 
entering into the formation of the ear-chamber is said to be developed 
from a special centre of ossification, No. 3 + (Fig. 9), and in accordance 
with the idea of it being the homologue of the epiotic centre of the 
human mastoid, named the epiotic. As there is no reason, however, 
for supposing that such a third centre of ossification exists, even if the 
part in question be characteristic of reptiles, the name epiotic should 
be discarded because it is misleading. Finally, as the author regards 
the bone No. 6, (Figs. 6, 7, 9, 10), not as the prootic but as the homo- 
logue of the alisphenoid in Man, (Figs. 1, 2, a), there is no reason for 
retaining the names prootic, opisthotic and epiotic.84 
Indeed, the periotic bones, or bone so named, should not be regarded 
as constituting any part of the proper cranial wall but as special 
ossifications of the ear-chamber depending upon the extent of the 
development of the organ of hearing. 
In a general way it may be said that the transitory conditions 
through which the human ear passes in the course of its develop- 
ment are more or less permanently retained as such in the organ 
of hearing in the lower vertebrata. Such being the case we cannot 
expect to find the protective osseous covering of the ear in the higher 
vertebrata equally well developed in the lower ones. On the con- 
trary, in proportion as the ear is undeveloped, we may expect to 
find any of the adjacent bones forming the wall of the cranium pro- 
tecting and entering into the formation of the ear-chamber, just as the 
tympanum is formed in birds by the basisphenoid, squamosal and 
ex-occipital rather than by the pars petrosa as in Man. There is no 
more reason for supposing that there is an archetypal temporal bone, 
34 The author does not refer to the skulls of birds, since the latter being 
specialized reptiles the disposition of the parts in question, as might be expected, 
is essentially the same. 
It should be mentioned, however, that the membrana tympani is not attach- 
ed in birds to the quadrate bone as in reptiles but to the outer margin of the tym- 
panum. 1894.] 
NATURAL SCIENCES OF PHILADELPHIA. 
51 
the different parts of which must exist in all vertebrata, than for 
believing that there is an archetypal vertebra and that the skull 
must consist of several of the same, at least in a Goethe-Oken sense. 
It does not follow, however, because a difference ot opinion may 
prevail among morphologists as to the special homologies of certain 
of the bones of the head that there should be any question as to the 
truth of the general doctrine of the unity of organization of the skull, 
so firmly established by Cuvier and others. 
On the supposition that the higher vertebrata have descended from 
the lower, it is to be expected that the general structure of the 
skull should be the same in both, the fundamental characters of the 
skull of the former having been acquired by inheritance from that 
of the latter. On the other hand, the skull should present greater or 
less modifications according to the special nature of the different 
vertebrata, such modifications being induced by the causes of varia- 
tion incidental to different kinds of life. 
The skull, like the organism in general, is not made according to 
a Platonic idea or pattern, but grows, its characters being acquired 
by inheritance as modified by variation. 52 
PROCEEDINGS OF THE ACADEMY OF 
[1894. 
Synonyms of Certain of the Bones of the Head According 
to French, German and English Anatomists. 
Cuvier (1). 
Meckel (2). 
Owen (3). 
Huxley (4). 
Occipital lateral. 
Seitliches unteres Hinter- 
hauptbein. 
Ex-occipital. 
Ex-occipital. 
2 
Occipital superieur (perch 
reptiles and birds). 
Hinterhauptschuppe. 
Superoccipital 
Supraoccipi- 
tal. 
3 
Occipital externe (perch, 
reptiles), apophyse mas- 
toid (mammals). 
Seitliches oberes, Hinter- 
hauptbein. 
Paroccipital. 
Epiot.ic in fish, 
Opisthotic in 
reptile. 
4 
Grand aile (fishes, birds, 
mammals), rocher (rep- 
tiles). 
Felsenbein (fishes and 
reptiles), Grosserkeil- 
beinfliigel (birds and 
mammals). 
Alisphenoid. 
Prootic. 
6 
Mastoidien (fishes and 
reptiles), temporal (birds 
and monotremes). 
Zitzenbein. 
Mastoid. 
Squamosal. 
8 
Aile orbitaire. 
Grosserkeilbe in flugel. 
Orbitospen- 
Alisphenoid. 
10 
Rocher (fishes, birds and 
mammals). 
Felsenbein (fishes). 
Petrosal. 
Opisthotic. 
16 
Jugal. 
Jochbein. 
Malar. 
Jugal. 
26 
Temporal (lizards, croco- 
diles and mammals), Jugal 
(birds, monotremes). 
Schlafenbeinschuppe. 
Squamosal. 
Quadrato-ju- 
gal. 
27 
Caisse (ophidia, croco- 
diles, mammals). Os tym- 
panique (lizards). Os car- 
re (birds). 
Pauke. 
Tympanic. 
Quadrate. 
28 
Temporal (fishes). Tym- 
panique (batrachia). 
Oberesgelenkbein. 
Epitympanic 
(fishes). 
Hyomandibu- 
lar (fishes). 
29 
Symplectique (fishes). 
Griffelformiges, Stiick 
des Schlafenbeins. 
Mesotympanic 
(fishes). 
Symplectic 
(fishes). 
30 
Tympanal (fishes). 
Scheibenformiges Stiick 
des Schlafenbeins. 
Pretympanic 
(ffshes). 
Metapterygoid 
(fishes). 
31 
Jugal (fishes and ba- 
trachia). 
Unteresgelenkbein. 
Hypotympan- 
ic (fishes). 
Quadrate 
(fishes) 
32 
(1) 
(21 
(3) 
(4) 
Lecons d'Anatomic Com- 
paree. 
Regne Animal. 
Histoire Naturelie des 
Poissons. 
Ossements Fossiles. 
System der vergleichen 
den Anatomie. 
Archetype of 
Vertebrate 
Skeleton. 
Anatomy of 
Vertebrates. 
Elements of 
Comparative 
Anatomy 
Manual of 
the Anatomy 
of Vertebrated 
Animals.