of the Museum of Comparative Zoology, 
AT HARVARD COLLEGE. 
Vol. XIII. No. 8. 
ON CERTAIN VACUITIES OR DEFICIENCIES IN THE CRANIA 
OF MAMMALS. 
By D. D. Slade. 
WITH TWO PLATES. 
CAMBRIDGE: 
PRINTED FOR THE MUSEUM. 
March, 1888. Sustain of ^omprafibt ^oolocjn, Cambridge, glass. 
With the compliments of 
Alexander Agassiz. No. 8. - On certain Vacuities or Deficiencies in the Crania of 
Mammals. By D. IX' Slade. 
Certain vacuities or deficiencies, ranging much in size, position, and 
relation to neighboring parts, exist in the macerated adult skull of the 
various orders of the Mammalia. 
These vacuities are in themselves due to an arrest of osseous develop- 
ment, and are in no wise to be confounded with the air cells in the can- 
cellous tissue, such as exist for example in the skull of the Elephant, 
or between the frontal plates of the Ox. These are secondary to the 
original growth of the bone. 
Neithei' are they, in any sense, the product of absorption. 
They occur for the most part at the juncture, or at what, under other 
circumstances, would be the juncture, of two or more bones, the margins 
of these bones thus becoming the boundary of the vacuity. They 
may also occupy the central portions of a bone, or they may hold the 
position of an ordinary foramen, or of two or more of these combined, 
thus representing, and even becoming in a true signification, enlarged 
foramina. 
The vacuities may therefore be arranged under two categories : - 
1. Those that are dependent upon arrested ossification, in the body 
of a bone, or at a point where several bones would otherwise come in 
contact, but neither of which has any special adaptation to function. 
2. Those that are due to enlarged openings, the result of arrested 
ossification, which have adaptation to special function, and retain this, 
notwithstanding the modifications which they may have undergone. 
The regions of the skull occupied by these vacuities may be thus 
classified : 1. Basal (posterior and lateral) ; 2. Orbito-nasal; 3. Pala- 
tine ; 4. Facial; 5. Occipito-squamosal; 6. Squamosal. 
1. Under the term "Basal" are included the posterior, lateral, and 
postero-lateral regions of the base of the skull, comprehending under 
this last that space existing between the posterior and middle cranial 
segments which in many cases is imperfectly filled by the periotic, 
tympanic, and squamosal, whereby deficiencies differing much in size 
are produced. 
VOL. XIII.-no. 8. 16 242 
BULLETIN OF THE 
These may be properly termed anterior and posterior tympanic vacu- 
ities, or foramina, corresponding to the foramen lacerum medium and 
foramen lacerum posterius basis cranii of anatomists. Plate II. Figs. 9, 
10, a. 
2. " Orbito-nasal " includes the vertical plate of the palatine, and 
the margins of the palatine, maxilla, alisphenoid, orbitosphenoid, and 
frontal, at their lines of juncture, as also the combination of certain 
foramina. Plate II. Fig. 7, a, b. 
3. " Palatine " designates the anterior and posterior portions of the 
palate, one or both. Plate I. Figs. 1, 2, a, b. 
4. "Facial" includes the side wall of the face, the latero-nasal por- 
tions of the maxilla, and the anterior root of the zygoma. Plate II. 
Fig. 8, a. Plate I. Figs. 3, 5, a, b. 
5. The " Occipito-squamosal " is the space comprehended between the 
supraoccipital, exoccipital, and squamosal. Plate II. Fig. 11. 
6. The " Squamosal " and " Parasquamosal " are indicated by the 
terms used. Plate I. Fig. 4, b. 
The cranial vacuities in the various orders of the Mammalia occur 
as follows. 
Monotremata. - The skull of the Echidna presents no vacuities. In 
the Ornithorynchus, there are relatively large anterior and posterior 
deficiencies, the first representing the foramen ovale, and the second 
the jugular and the prsecondyloid foramina combined. There are also 
small ones in the basisphenoid. 
Marsupialia. - Many of the Macropodidse have large posterior palatine 
vacuities. These are present also in Phascolarctos (Plate I. Fig. 1, a), 
in the Dasyuridse (Plate I. Fig. 2, a, b), and in the Peramelidce. Pera- 
meles lagotis has a large oval vacuity which extends from the second pre- 
molar to the penult molar, and posterior to this are found several small 
ones. In the Didelphidse there are large posterior palatine vacuities. 
Edentata. - This order is singularly free from cranial deficiencies. 
In the Dasypodidse the posterior tympanic vacuity, or foramen lacerum 
posterius, is somewhat enlarged. Among the Sloths, Choicepus hoffmanni 
presents considerable deficiencies in the basi-sphenoidal region. 
Chiroptera. - In a few genera, notably in Pteropus, the posterior 
tympanic vacuities are large. 
Insectivora. - In some of the Erinaceidse, the post-tympanic are 
large, while there are also extensive post-palatine vacuities, especially in 
Erinaceus europceus. In Sorex, large latero-basal ones are found, and 
Tupaia has a large longitudinal central deficiency of the malar, in addition 
to the palatal vacuities. MUSEUM OF COMPARATIVE ZOOLOGY. 
243 
Rodentia. Lagomorpha. - In the Hare, at the posterior portion of 
the inter-orbital septum, the foramen, which serves as a common outlet 
of the optic nerves, is sufficiently enlarged to constitute a vacuity. 
There are also large vacuities extending from each orbit into the 
latero-nasal regions; these being covered externally by the singularly 
reticulated plate of the maxilla. (Plate I. Fig. 3, a, b.) Pre- and post- 
palatine deficiencies reduce the palate itself to a mere narrow bridge 
extending across between the premolars. 
Sciuromorpha. - In the Sciuridae, there is a small orbito-nasal defi- 
ciency. In Castor fiber there are large anterior tympanic spaces. 
Myomorpha. - The anterior root of the zygoma presents a vacuity 
in very many of the Rodentia (Plate I. Fig. 4 a), and in the Rats gen- 
erally this is vertical and dilated superiorly. In Fiber zibethicum, the 
malar vacuity is large, and there is also one in the posterior process of 
the squamosal (Plate I. Fig. 4 6), which is also present in several of 
the Muridae. In Lagostomus, a thin bony plate separates the infra- 
orbital foramen from the antorbital vacuity. 
Hystricomorpha. - In the Porcupines generally, and especially in 
Erethison dorsatus, the infra-orbital as also the ante-tympanic vacuities 
are large. In the Capybara, the former is immense. In Chinchilla a 
large deficiency also occurs in the parasquamosal region between the 
alisphenoid, parietal, and tympanic. (Plate I. Fig. 5, a.) In Paca, the 
foramina, rotundum, and lacerum anterius combine to form a capa- 
cious orbito-nasal vacuity. 
Carnivora. Pinnipedia. Phocidae. - Large latero-posterior basal de- 
ficiencies occur in all, while in some, as in P. vitulina, P. groenlandica, 
and P. foetida, there is in addition to these a more or less extensive per- 
foration in the basioccipital. (Plate I. Fig. 6, b.) In the orbito-nasal 
region, not only is the spheno-palatine foramen much enlarged, but a 
deficiency occurs at the juncture of the maxilla, palatine, and frontal, 
often attaining a large size. (Plate II. Fig. 7, a.) 
Otariidce. - The foramen la,cerum posterius is much enlarged, while 
the orbito-nasal vacuities are extensive, in this family. In Zalophus 
and Callirhinus, in addition to the above, there are large vacuities at 
the juncture of the vertical plate of the palatine with the alisphenoid and 
orbitosphenoid. (Plate II. Fig. 7, b.) 
Rosmaridce. - In the Walruses there are large orbito-nasal and pos- 
terior tympanic vacuities, while the anterior root of the zygoma is 
largely dilated. 
Fissipedia. - In this sub-order, with very few exceptions, no vacuities 244 
BULLETIN OF THE 
occur. In Enhydra (Sea Otter) the post-tympanic fissure is large; 
there is also an orbito-nasal deficiency. The infra-orbital opening is 
capacious. 
Ungulata. Artiodactyla. - In many of the Ruminants, as in the Cer- 
vidse, Camelidse, Antilocapridaa, and in some of the Bovidse, as in Anti- 
lopinae and Caprinae, a large facial vacuity exists at the juncture of the 
frontal, lacrymal, malar, and nasal bones, which leads into the nasal 
cavities. (Plate II. Fig 8, a.) Where the upper incisors are entirely 
absent, the anterior palatine foramina assume large proportions, becom- 
ing veritable deficiencies. In some of the Cervidae, as in Rangifer, 
Alces, and in Antilocapridae, a more or less extensive orbito-nasal vacuity 
exists at the juncture of the vertical plate of the palatine, alisphenoid, 
and lacrymal. In Suidae and Dicotylidae the foramen lacerum medium 
is large, and in Hippopotamidae both ante- and post-tympanic foramina 
are capacious. 
Perissodactyla. - In the Equidae, the anterior and posterior tympanic 
foramina with the ovale combine to form an extensive deficiency in the 
postero-lateral basal region. (Plate II. Fig. 9, a.) In the Rhinoce- 
rotidse, the foramina, ovale, and lacerum medium are combined. There 
is also a considerable orbito-nasal vacuity, as well as one formed by 
the combination of the two anterior palatine foramina. 
In Tapiridae, the anterior, posterior tympanic foramina, and the ovale 
are joined, as also are the anterior palatine, as in the Rhinocerotidae. 
Sirenia. - In the two genera Manatus and Halicore, which consti- 
tute the present order, the entire latero-posterior basal region, between 
the occipital and alisphenoid, may be considered as a vacuity, so imper- 
fectly is it filled by the tympanic and periotic. (Plate II. Fig. 10, af 
In the later-ooccipital region, at the juncture of the squamosal with 
the supraoccipital and exoccipital, a large vacuity also exists, very im- 
perfectly filled by the periotic. (Plate II. Fig. 11, a, b.) The dilatation 
of the supraorbital foramen is also large, and there are extensive orbito- 
nasal openings. 
Hyracoidea. - In Hyrax, the anterior tympanic deficiency is rela- 
tively large. 
Proboscldea. - In the Elephant, the tympanic deficiencies so uni- 
formly present in the odd-toed Ungulates can scarcely be said to exist. 
Cetacea, - In the Odontoceti, and notably in the Delphinoidae, large 
irregular openings exist between the recess which holds the united 
tympanic and periotic and the cranial cavity. The optic foramen, as 
it passes out through the orbito-sphenoid, is much enlarged, being MUSEUM OF COMPARATIVE ZOOLpGY. 
245 
converted in some instances into a vacuity. Upon the latero-external 
walls of the posterior nares large oval notches or deficiencies occur. 
Primates.-The sphenoidal and sphenomaxillary fissures in man, as 
also the foramen lacerum medium, which is often very large and irregular 
in shape, come under the second category of vacuities. The same also 
applies to the anterior tympanic of the higher Apes. 
In studying the etiology of these cranial vacuities in the Mammalia, 
it would seem at first sight, especially if the imperfect osseous condition 
of the skull in many of the lower Vertebrates be taken into consideration, 
that their existence was due to phylogenetic degeneration, taken in 
the widest acceptation of the term. A closer study of the conditions 
presented, however, apparently limits this degeneration to one of en- 
vironment. Take as an illustration the Pinnipedia, in which group 
the economy of nature as regards the disposition of material is admira- 
bly shown. These true Carnivora have become adapted to an exist- 
ence in water. Consequent changes suited to this aquatic life have 
been undergone. The thick skull of the Fissipedia is no longer neces- 
sary ; consequently the cranial walls have become much thinned, and 
several large and extraordinary vacuities in different regions, notably 
in the basal, have been formed. 
A reduction has taken place in the number and strength of the teeth, 
and other characteristics of the order have been materially altered or 
entirely lost. These changes have been foreshadowed in the Enhydra 
(Sea Otter), as has already been noted. 
So also any diminution in the strength of the cranial walls of the 
ancient branch of the Ungulata, the Sirenia, due to large deficiencies, 
is counterbalanced by the aquatic habits of the order. 
In the Rodents, the alveolar border of the maxilla is pushed far back, 
and thus the unoccupied space between the incisors and premolars is 
relatively large. This absence of teeth necessitates only a small supply 
of bony material in the immediate region, so that in the case of the 
Hare, which feeds upon a succulent diet, little strength or resistance is 
demanded, arid consequently large vacuities are found in the latero-nasal 
region. The nasal plate of the maxilla is rendered lighter by the 
reticulation, which may be considered as a series of small vacuities, 
while it still serves as a protection to the delicate structures beneath. 
Again, the presence of the vacuity in the side-walls of the face in many 
of the Ruminants, and in consequence the apparently weakened condition 
of the parts, may be explained by the statement that either the animal 
is of so peaceful a nature that consolidated bones such as the frontals 246 
BULLETIN OF THE MUSEUM OF COMPARATIVE ZOOLOGY. 
of the Bovinse are entirely unnecessary, or, being combative, provision 
is made for purposes of offence and defence by the growth of antlers 
or other horns. 
Thus, without further illustration, it may be said, that while it is 
difficult in the present state of our knowledge to account for the varied 
character and position held by cranial vacuities, or to explain why these 
should be more frequently present in some orders than in others, and 
in certain genera of an order and not in others, it may nevertheless be 
safely affirmed that whenever osseous material can be set aside without 
interference with the general economy, or without detriment to the 
structure of the immediate parts, it is dispensed with. This general 
law applies to all vacuities or deficiencies, wherever situated, in the 
crania of the Mammalia. 
Cambridge, January, 1888. SLADE. CRANIAL VACUITIES. 
PLATE I. 
Boston Photogravure ( o. SLADE. CRANIAL VACUITIES. 
PLATE II.