ON THE 
PHYSIOLOGICAL ACTION 
« 
, * f 
OF THE 
MUSCLES CONCERNED IN THE MOVEMENTS 
OF 
THE LOWER JAW. 
By Thomas Brian Gunning. 
E PUR 81 MUOVE. 
SEE 
No. XXXIII. of the NEW-YORK MEDICAL JOURNAL, Vol. VI., 1867, 
ALSO, 
No. LXIII. of the NORTH AMERICAN JOURNAL OF HOMOEOPATHY, 
Vol. XVI., 1868. 
NEW-YORK: 
PRINTED BY H. LUDWIG, 39 CENTRE-STREET. 
1868. ON THE 0F THE \ 
( MASSACHUSETTS 
PHYSIOLOGIC ALqACTIDN 
OF THE 
MUSCLES CONCERNED IN THE MOVEMENTS 
OF 
THE LOWER JAW. 
By Thomas Brian Gunning, 
E PUR 81 MUO YE. 
SEE 
No. XXXIII. OF THE NEW-YORK MEDICAL JOURNAL, Vol. VI., 1867, 
ALSO, 
No. LXIII. of the NORTH AMERICAN JOURNAL OF HOMCEOPATIIY, 
Vol. XVI., 1868. 
• NEW-YORK: 
PRINTED BY H. LUDWIG, 39 CENTRE-STREET. 
1868. 1. Centre of the condyle upon which the head rocks. 2. Styloid process. 3. Condyle of 
Lower Jaw. 4. Neck of do. 5. Angle of do. 6. Eminentia Articularis. 7. Curve upon which 
the jaw opens. 
A. External Pterygoid. B. Upper head of do. C. Lower head of do. D. Genio-Hyo- 
Glossus. E. Genio-Ilyoid. E. Mylo-IIyoid. G. Ilyo-Glossus. II. Middle Constrictor. J. 
Inferior do. K. Thyro-Hyoid. L. Sterno-Tliyroid. M. Stylo-Hyoid. N. Stylo-llyoid Liga- 
ment. O. Stylo-Maxillary Ligament. P. Complexus. 
The chin is thrown up and the jaw and the curved line which indicates the position of the 
condyles of the head, are drawn small to show the muscles more distinctly. For the same pur- 
pose the Platysma-myoides and Masseter muscles are left out, and the insertion of the Temporal 
and the origin of the Trapezius cut away. ON THE SOCA^ 
PHYSIOLOGICAL ACTION 
OF THE 
MUSCLES CONCERNED IN THE MOVEMENTS 
OF 
THE LOWER JAW. 
[We extract the following important article from an extended Memoir by Dr. 
Gunning, on the Muscles of the Head, Neck, Jaw and Palate. This paper is 
introductory to one on “ Diagnosis of Fractures of the Lower Jaw,” which was 
promised in his article, published in the September number of the New-York 
Medical Journal, 1866, and also in this Journal, May, 1867. The paper on 
Diagnosis will appear as promised. [Ed. N. A. Jour.] 
The necessity for muscles of great power, and acting upon long 
levers, to turn the head quickly, is demonstrated by the ac- 
tion of the sterno-cleido-mastoid. In very quick turning of 
the head the muscle acts instantaneously; this, however is but 
seldom. In the ordinary rotation of the head, it takes no 
part whatever, unless the head is obstructed as when lying down 
on the side. But if the head is turned far around, the muscle 
always acts firmly in the last part of the movement. This 
can be verified if the body is held upright and the forefinger 
placed in the interclavicular notch, with the thumb and second 
finger resting on the tendons of the muscles, and notice taken 
of the tightening and relaxation of the tendons. The com- 
parative indifference of this muscle to the head's rotation can 
be more easily demonstrated in the evening, or -when fatigued. 
From this it appears that the action of the sterno-mastoid in 
turning the head is of a very secondary character. It acts 
only when the rotators which pass from the axis to the atlas 
and occipital bone, and the splenius capitis and colli of the 6 
opposite side, are already in action, and even then only to as- 
sist in turning the head quickly, to carry it further round, or 
to overcome obstruction. When it assists in turning the head 
it draws one mastoid process forward, while the splenius pulls 
the other mastoid process backward. 
The sterno-cleido-mastoid muscle is said to be a rotator, a 
flexor, and an extensor of the head. What this flexing of the 
head means, in addition to lateral movement, may be learned 
by the following quotation. “The sterno-mastoid muscles, 
when both are brought into action, serve to depress the head 
upon the neck, and the neck upon the chest.”* These views 
are also maintained by J. Cruveilhierf and may be accepted 
as those not only of the French anatomists and physiologists 
generally, but also of the German and English with few ex- 
ceptions. Professor Henle, however, says positively that these 
muscles do not flex the head down in front, and that they lift 
the head and bend the neck when the body is brought up in ris- 
ing from the back.J This is a great advance upon what is said 
by others, but beyond this he gives no intimation of under- 
standing their peculiar and most important function. 
The insertion of the sterno-cleido-mastoid muscle is around 
the front of the mastoid process, and back along the superior 
curved line, about half the distance between the mastoid pro- 
cess and the centre of the occipital protuberance, while the 
front of the mastoid process is nearly always on a line with 
the centre of the condyles of the occipital bone (in rare in- 
stances, however, it is nearer the front of the condyle). The 
sterno-cleido-mastoid muscle is consequently inserted back of 
the centre upon which the head rocks (except in rare cases 
when a small portion of the muscle is a little forward of it). 
Notwithstanding this, it is set down as rocking the head for- 
ward, and the action of the muscle in rising is brought forward 
to prove it. This experiment, however, if properly conducted 
•Gray’s Anatomy. 2d Amer. edit., p. 256. Phil. 1865. 
t Traite d’Anatomie Descriptive. Troisieme edit., tome deuxieme, p. 173. 
Paris. 1851. 
% Handbnch dcr Muskellehre des Menschen von Dr. T. Henle, Professor der 
Anatomie in Goettingen. (Page 110). 7 
and explained, will prove the contrary. If the experimenter, 
while lying flat on his back, with the forefinger resting in the 
interclavicular notch, and the thumb and second finger on 
the tendons, will raise his head and shoulders a little, he will 
find that the muscles are acting strongly ; then by staying in 
that position and rocking the head backward and forward, it 
will be felt that the muscles are unaffected in any part of 
their fibres, and that they pay no attention to the movement 
of the head, neither the tendons on the sternal portions nor 
those on the clavicular being relaxed for a moment. Then sit 
up, throw the head forward sufficiently to relax the tendons, 
and rock the head as before ; it will now be found that the 
tendons remain relaxed, showing that tightness of the tendons 
did not conceal action of the muscles in the first experiment, 
and demonstrating that the sterno-cleido-mastoid muscles do 
not “ serve to depress the head upon the neck.” In bringing 
the head forward these muscles act only until the head 
comes to its centre of balance, when the tendons relax 
and remain so, even when the chin touches the breast. 
But if the head is obstructed in this downward move- 
ment, these muscles will then assist to bring it down in 
front and to hold it there. The sterno-cleido-mastoid mus- 
cles do not, however, in this rock the head upon the atlas, 
but bring and keep the atlas forward. Neither are they 
“ extensors of the head” in the sense indicated by the books, 
which seems at first sight to accord more with their insertion 
back of the centre of the condyles. But the insertion is so 
peculiar that it requires consideration to determine how the 
muscles affect the head. The mastoid process is always below 
the superior curved line upon which the back part of the mus- 
cle is inserted. When the process is large it may be more 
than an inch below it, although much less when the process is 
small, as in childhood before the cells are developed. More- 
over, the uniformity of position between the mastoid processes 
and the condyles horizontally is not met with in their vertical 
relation, the condyles being on some skulls more than half an 
inch lower than the mastoid processes, while on others the pro- 
cesses are as much below the condyles, the large proportion 
being between these extremes. These variations go far to show 8 
that the sterno-mastoid muscles are not intended to rock the 
head backward, for when the mastoid process is much lower 
than the condyles, and especially when it is large and projects 
forward somewhat, to correspond to the direction of the muscle 
it follows that as the head is pulled downward (by the trape- 
zii, &c.) the mastoid processes go upward and forward ; conse- 
quently if the stei'no-cleido-'niastoid were to act to bring the head 
down behind, the portion on the mastoid process—the strongest 
part of the muscle—would hold the head down in front, pro- 
bably as much as that on the occipital bone would pull it 
down behind. But their action can be tested by lying down 
so as to remove the necessity for action of the muscles to hold 
the atlas. In this position (care being taken not to lift the at- 
las, or neck) the sternal portion of the muscles will not act 
in concert with the other muscles to rock the head back, even 
if the whole weight of the body is thrown upon the back of 
the head, and I have been unable to find any action in the 
clavicular portion, although the action of this part of the mus- 
cle is so delicate and prompt that it can be distinctly felt when 
the foot is raised in walking, the head and body being then 
thrown over to the other side to restore the balance. Further, 
when the sterno-cleido-mastoid and the splenius of the same 
side are acting in concert to pull the head down to the 
shoulder, no backward movement of the head is discoverable. 
This is conclusive, for both these muscles having similar inser- 
tions, if one rocks the head back the other must, and their 
combined action would be manifest if they exerted it. 
It has been previously shown that this muscle acts as a 
rotator only by sometimes assisting the splenius, &c., of the 
opposite side, and as a lateral flexor, in connection with the 
splenius of the same side, but only when the head is ob- 
structed, and then generally by its clavicular portion, the 
sternal acting only in extreme necessity. It is now seen that 
it does not flex the head down in front, that is upon the 
atlas, and that its action as an extensor of the head can not 
be demonstrated. The proper function of the sterno-deido- 
mastoids when acting in concert, is to give anterior support to 
the top of the spine, the splenii muscles giving posterior sup- 
port. This may be easily proved by sitting down and watch- 9 
ing the tendons. When the head is back of its centre of 
support both the sternal and clavicular tendons are tightened, 
when rising they become tenser until the head is started, as it 
comes into balance they relax. On sitting down, the tendons 
tighten to check the head as it goes back out of balance. 
Sudden forward movements tighten them until the head is in 
motion, they then slacken as the head is forward of the centre 
and the atlas supported by the splenii muscles. If the head 
is in balance, any pressure upon the forehead acts with increased 
force upon the atlas and brings the muscles into action to keep 
it upright. The action of the sterno-cleido-mastoid muscles in 
these movements is but a modification of the service rendered 
by them in raising the head from the horizontal position, in 
doing which the muscles at first support more than the weight 
of the head, for in supporting the mastoid processes they sup- 
port the atlas, and make it a fulcrum between the bulk of the 
head and the counter-balance at the other end of the lever, but as 
the body comes upright and the head into balance, the strain 
upon the sterno-mastoid muscles gradually diminishes, until 
the head is held by the posterior muscles, when the atlas bears 
all the weight vertically. 
[A reference to the figure will render this explanation more apparent. The same figure also 
illustrates the action of the muscles of the lower jaw, and confirms the opinions expressed 
in the subsequent portions of this paper.] 
The hyoid bone, in addition to the muscles which pass to it 
from parts above the lower border of the jaw, gives attachment 
to others, which pass up the front of the neck below the jaw. 
Of these the sterno-thyroul arises close to the centre of the 
posterior surface of the upper bone of the sternum, and falling 
back somewhat as it passes up, is inserted into the side of the 
thyroid cartilage, from whence the thyro-hyoid (appearing 
like a continuation of the preceding) goes up and is inserted 
into the body and greater cornu of the hyoid bone. The 
sterno-hyoid arises from the sternum and end of the clavicle 
and is inserted into the lower border of the body of the hyoid 
bone. It is separated considerably from its fellow at its origin, 
but crosses the sterno-thyroid and approaches it in the middle 
of its course; it leaves the front of the thyroid cartilage un- 
covered. 10 
The omo-hyoid arises from the upper border of the scapula, 
and occasionally from the transverse ligament which crosses 
the supra-scapular notch. It passes across and up the side of 
the neck to be inserted into the body of the hyoid bone. It 
crosses under the trapezius and sterno-cleido-mastoid muscles 
hut over the scaleni and thyro-hyoid. It is a double-bellied 
muscle united by a tendon which is held down by a process 
of the deep cervical fascia. The first portion is nearly hori- 
zontal in its course, but underneath the sterno-mastoid muscle, 
where the cervical fascia passes around the tendon, it turns up 
so that the second portion is nearly vertical in its course to 
the hyoid bone. These are the directions of the muscle when 
at rest, but when active it approaches the line of its attach- 
ments and the cervical fascia is drawn upward and backward. 
The digastric, another double-bellied muscle, has peculiar 
relations with the preceding. It arises from the digastric 
notch, on the inner side of the mastoid process of the temporal 
hone, and passes downward, forward, and inward, to the side 
of the hyoid hone, where its rounded tendon (after passing 
through the stylo-hyoid muscle) is held hy an aponeurotic 
loop in connection with the side of the body of the hyoid 
hone above the insertion of the omo-hyoid. The muscle then 
passes forward and is inserted into a large depression on the 
inner side of the lower border of the jaw close to the symphy- 
sis. The tendon which divides the posterior and longer belly 
from the anterior, gives off a large aponeurotic layer, which is 
attached to the body and great cornu of the hyoid bone; and 
with the portion on the opposite side is termed thq supra-/,yoid 
aponeurosis, which forms a strong layer of fascia between the 
anterior portions of the two muscles, and a firm investment 
for the other muscles of this region. The digastric muscle is 
peculiar in not being inserted into the hyoid bone, but at- 
tached to it by a loop ; this allows the muscle to act without 
interfering too much with the hyoid bone. The muscle has 
not, however, that freedom which is attributed to it as a re- 
flected cord, for its aponeurotic connection with the hyoid 
bone and adjoining muscles prevents it from sliding through 
the loop which attaches it to the hyoid bone, except to a very 
limited extent. This powerful muscle exerts great influence 11 
from tlie various and important movements in which it takes 
part. 
The last muscle to he described in this connection, the pla- 
tisma myoides, is very distinctly separated from all the others. 
It is a broad thin plane of muscular libres, immediately be- 
neath the skin, on the side of the neck. It arises from the 
clavicle and acromium, and from the fascia covering the upper 
part of the pectoral, deltoid and trapezius muscles, and going 
upward and forward, it covers in the angle and the border of 
the jaw to the symphysis. It is inserted into the lower border 
of the jaw, in front, but back of the commissure of the lips it 
is found interlaced with the muscles above. It affords mus- 
cular support to the integument, and a cover to the muscles 
beneath, but leaves the thyroid cartilage and the front of the 
trachea free. 
The service supposed to be rendered by the foregoing mus- 
cles is shown by the following selections: 
J. Cruveilhier says: “The sterno-hyoid, the omo-hyoid, the 
sterno-thyroid, and the thyro-hyoid are the simplest in their 
structure and the simplest in their action ; all cooperate to the 
lowering of the jaw. Moreover, if the jaw is fixed, they flex 
the head.”* 
Sappey says: “The genio-hyoid is the elevator of the hyoid 
hone when the jaw is fixed; lowerer of the jaw if the hyoid 
bone is motionless; flexor of the head when the hyoid bone 
and jaw are both fixed.”f 
J. Cruveilhier says of the digastric : “ If the hyoid bone is 
fixed the posterior belly becomes the lowerer of the jaw, in 
consequence of the reflection of the muscle; the anterior and 
posterior bellies can throw the head backward.”:}: 
Jamain says: “If the hyoid bone is fixed, the digastric co- 
operates in lowering the jaw.”|| 
Todd’s Cyclopaedia says: “ When the hyoid bone is fixed 
by its depressors, and perhaps in some degree retracted by 
♦ Traite d’Anatomie Descriptive. 3me ed., tome 2me, p. 179. Paris, 1851. 
+ Traite d’Anatomie Descriptive. Tome 1, premiere partie, p. 213. Paris, 
1850. 
X Traite d’Anatomie Descriotive. Troisieme edit., tome deuxieme. p. 182. 
Paris. 1851. 
ii Nouveau Traite Elementaire d’Anatomie Descriptive, p 180. Paris, 1853 12 
the joint actions of the posterior belly of the digastric and of 
the omo-hyoid, the anterior belly, both passively as a reflected 
cord, and actively in virtue of its muscular fibres, depresses 
the lower jaw, and opens the mouth.”* 
“ Chief action of the omo-hyoids is to tighten the cervical 
fascia during deglutition ; they are also capable of depressing 
the hyoid bone.”f 
Gray’s Anatomy says of the digastric, mylo-hyoid, and genio- 
hyoid : “When the hyoid bone is fixed by its depressors and those 
of the larynx, they depress the lower jaw and further, that 
in deglutition “ the anterior belly of the digastric carries the 
hyoid bone, &c., upward and forward and the posterior belly 
upward and backward,” and says of the platysma-myoides: 
“ Its anterior portion, the thickest part of the muscle, depresses 
the lower jaw.”|| 
Henle thinks : “ The platysma-myoides are not depressors 
of the lower jaw.Ӥ 
Duehenne says: “ Its action being exhausted by the mo- 
bility of the integuments of the face, the neck and the chest, 
it has no longer sufficient strength to depress the lower jaw.”*!" 
Cruveilheir says: u The platysmas are sometimes unequal 
in strength.”** 
Ziemssen says: “ The muscle is sometimes absent.”ff 
The absence of the platysma-myoides in some cases, and its 
inequality in others, proves that it is not of any consequence 
in depressing the jaw, which is a movement requiring great 
promptness and exactitude. It may be held between the 
thumb and finger, near the front of the jaw, and if care is 
taken to discriminate between it and the integument, it may 
be felt that the muscle pays no attention to the movement of 
the jaw. 
* Todd’s Cyclopaedia. Yol. III., p. 564. 
t Ibid. p. 563. 
t Gray’s Anatomy. 2d American Edition, p. 260. 
II Ibid. p. 256. 
§ Handbuch der Muskellehre des Menschen, von Dr. T. Henle, Professor der 
Anatomie in Goettingen, p. 108. 
IT De l’Electrisation Localisee. p. 380. Paris, 1855. 
** Traite d’Anatomie Descriptive. Troisieme edit., tome deuxieme. p. 166. 
Paris, 1851. 
ft Die Electricitaet in der Medicin, p. 44. Berlin, 1857. 13 
The muscles which centre in the hyoid hone have power to 
control and move the organs to which they are attached, (and 
of which they are in several instances important parts,) sub- 
ject, however, to the following limitations. The stylo-hyoid 
ligament passes down on each side from the styloid process of 
the temporal bone to the little horn of the hyoid bone. These 
ligaments have, therefore, a slanting course, while the supra- 
hyoid aponeurotic layer, between the hyoid bone and the inner 
side of the front of the jaw, has a horizontal direction. By 
this arrangement the glottis and its covering, &c., are held at 
some distance from the back of the pharynx, and free respira- 
tion secured, independent of muscular action, wdiile the hyoid 
bone can move upward or forward to a considerable extent, 
and be returned to its natural position when at rest, in any 
direction that is not back of this resting-place. But below 
this the downward, and especially the backward movements 
of the hyoid bone are very limited, being only what is gained 
by the tightening of the ligaments, t%c. 
Although a case is sometimes seen in which the stylo-hyoid 
ligaments give place to muscles, in others they are entirely ossi- 
fied, and the temporal bones and the hyoid bone are in one piece. 
Showing that, depression of the hyoid below its natural posi- 
tion when at rest, is unnecessary, except to a trifling extent. 
The digastric muscle is set down as drawing the hyoid bone 
backward and forward in deglutition, and as depressing the 
jaw by acting as “ a reflected cord/’ These services are incon- 
sistent with each other and with the anatomy of the parts. 
If it were fixed so as to draw the bone backward and forward, 
it could not slide and be of service as a “ reflected cord ” suffi- 
ciently to lower the jaw. To do the latter the anterior belly 
should be inserted higher up the jaw, while a long unrestricted 
tendon of the muscle should run through a fixed loop on the 
lower border of the hyoid bone, which last should also be freed 
from the styloid ligaments, and be drawn down half-way to the 
sternum every time the jaw opened wide, and proportionally 
for less opening. 
In respect to the united action of both bellies drawing the 
head backward, it is only necessary to say that the origin of the 
digastric is partly in front of a line drawn across, just behind 14 
the condyles of the occipital bone; it could not, therefore, 
draw the head back appreciably even if its insertion were di- 
rectly under its origin. It is consequently a mistake to sup- 
pose it can do so when its direction forward is more horizontal 
than vertical. In fact this muscle is the great agent in draw- 
ing the head forward. The posterior belly slants down to the 
hyoid bone, but the anterior is nearly horizontal in its course, 
and when the muscle acts it tends to the line of its attach- 
ments by drawing or endeavoring to draw the hyoid bone 
upward unless the jaw is much depressed, when, as the muscle 
is straight, or nearly so, it has no power to raise the hyoid 
bone. But in several important services the digastric acts in 
concert with the omo-hyoid. In this way the muscles passing 
from the hyoid bone to the front of the jaw, including the 
anterior belly of the digastric, are as effectually antagonized 
as if a powerful muscle passed from each side of the hyoid 
bone to the opposite cervical vertebrae, with the advantage of 
greater length of muscle to contract, and easier adaptation to 
the movements of the jaw; and the muscles in front of the 
hyoid bone act, when necessary, in alternation with the omo- 
hyoid and the posterior belly of the digastric. More fre- 
quently, however, the anterior belly of the digastric acts with 
the posterior belly and the omo-hyoid, for they keep the head 
upright. In doing this the omo-hyoid muscle and the posterior 
belly of the digastric draw or hold the hyoid bone back, while 
the anterior belly of the digastric brings in the chin, and the 
temporal and other elevators of the jaw draw the head forward; 
in this way the digastric acts on a long lever, as the head rocks 
on a centre, but a little below the entrance of the external ear. 
The digastric and omo-hyoid muscles are always active during 
forward or backward movements of the body or head. They 
do for the head what the sterno-mastoid muscles do for the 
spine, and their action can be felt easily with the finger, in 
sitting down or rising up, &c. They are also powerful rota- 
tors of the head, and the action of the omo-hyoid is singularly 
quick in sudden turns of the head, (as with the sterno-mastoid 
muscles,) the digastric being useful in assisting to keep the 
hyoid bone up in place, it being held laterally by the apo- 
neurosis and probably by the mylo-hyoid muscle. 15 
If the end of the finger is placed just behind the origin of 
the cleido-mastoid during these movements, the omo-hyoid 
will be felt rising above the clavicle, and carrying the cervical 
fascia upward and backward ; and if a finger is placed behind 
the mastoid process so as to cover the end of the digastric 
notch, the digastric muscle will be felt acting in concert with 
the omo-hyoid, and the anterior belly can be felt between the 
jaw and hyoid bone. The peculiar attachment of the digastric 
can now be appreciated, as the hyoid bone is left sufficiently 
free in its various movements, although it is at the same time 
the centre of control and support to the head. The importance 
of this support to the head can hardly be over-estimated, for 
the weight of the head beyond the atlas must be balanced. 
This the digastric and omo-hyoid muscles do effectually by 
acting upon the jaw, which is a lever whose length below the 
top of the atlas is over one-third of the height of the head 
above the atlas. The points from which these muscles act are 
the mastoid process and the shoulder; the vertex of their 
angle being in the hyoid bone, from whence they draw in the 
chin; in this direction they are very active and powerful. 
They not only balance the head in locomotion and leave the 
other muscles free to act in deglutition, vocalization, and arti- 
culation, but frequently cooperate with them. 
The following quotations show the opinions entertained as 
to the action of the muscles which move the lower jaw: 
Gray’s Anatomy says: “ The temporal masseter and internal 
pterygoid raise the lower jaw against the upper with great 
force. The two latter museles, from the obliquity in the direc- 
tion of their fibres, assist the external pterygoid in drawing 
the lower jaw forward upon the upper, the jaw being drawn 
back again by the deep fibres of the masseter, and posterior 
fibres of the temporal. The external pterygoid muscles are 
the direct agents in the trituration of the food, drawing the 
lower jaw direetly forward, so as to make the lower jaw pro- 
ject beyond the upper. If the muscle of one side acts, the 
corresponding side of the jaw is drawn forward, and the other 
condyle remaining fixed, the symphysis deviates to the oppo- 
site side. The alternation of these movements on the two 
sides produces trituration.”* 
* Gray’s Anatomy, Descriptive, and Surgical. 2d American Edition, p. 252. 
Phila. 1865. 16 
Todd & Bowman’s Physiological Anatomy, part iii., p. 539, 
says: “ The external pterygoid neither raises nor depresses the 
lower jaw.” 
My own views as to these muscles also, differ materially on 
some points from those expressed in the quotations. The 
stylo-hyoid ligaments make it impossible that the hyoid system 
of muscles can depress the lower jaw by acting upon it verti- 
cally, and the following experiments show that they do not. 
By resting the finger on the thyroid cartilage, with its end 
placed against the hyoid hone, it will he found that they do 
not descend when the jaw is opened.* Further, if the thumb is 
placed under the chin and the jaw held firmly open against 
it, hy carefully throwing the hyoid hone up hy swallowing, it 
will he found that the jaw is held down hy other muscles than 
those inserted into the hyoid bone; as it will not go up if the 
experiment is properly conducted, even when the hyoid hone 
is carried above the border of the jaw. This may, howmver, 
require some care and practice; as the jaw is, in general, nearly 
shut in deglutition, and its depressors are inclined to relax in 
sympathy with the movements of the other parts. The position 
of the masseter is too well known to need particular descrip- 
tion. The fibres of the deep portion have a more perpendi- 
cular direction than those of the superficial portion, the last 
passing backward as much as downward. The deep portion 
draws 1 he jaw upward and backward, the superficial portion 
upward and forward. The internal pterygoid has the same 
general direction as the superficial portion of the masseter, 
excepting that as it arises from the pterygoid fossa it passes 
considerably outward to reach its insertion on the inner side of 
the ramus and angle of the jaw. It has, therefore, not only 
the upward and forward motion of the superficial portion of 
the masseter, but also lateral power over the jaw. 
These muscles not only raise the jaw and teeth, when cut- 
ting with the incisors and crushing with the molars, but are 
also the main movers of the jaw in trituration. In cutting 
they bring the jaw forward bodily; in triturating they exert 
more forward and lateral action on one side than on the other; 
by this the jaw is thrown over to the opposite side and then 
* Except to a trifling extent, when the jaw is opened unusually quick or 
wide 17 
drawn in and carried out continuously on that side, and not 
carried over to the other side. It is a mistake to suppose that 
trituration of the food is effected by the alternate action of the 
muscles of both sides. When the jaw and teeth are perfect, 
the teeth of one side only are used, until the muscles tire, per- 
haps, and then those of the other side are resorted to. If the 
teeth are tender, badly placed, or deficient on one side, tritura- 
tion is performed on the other only. 
The temporal muscle, which covers so large a portion of the 
side of the head, and is strongly inserted into the inner surface, 
apex and anterior border of the coronoid process of the jaw, 
pulls its insertion upward and backward, and assists the mas- 
seter and internal pterygoid muscles in cutting, crushing and 
triturating the food. 
The external pterygoid is a short, thick muscle, which arises 
from the pterygoid ridge on the great wing of the sphenoid, 
and the portion of bone included between it and the base of 
the pterygoid process, from the outer surface of the external 
pterygoid plate and the tuberosity of the palate and superior 
maxillary bones. It arises in two portions separated by a 
short interval; they both pass outward and backward and are 
inserted into a depression in front of the condyle of the lower 
jaw, and into the corresponding part of the interarticular fibro- 
cartilage. The separate portions join and form one muscle 
previous to their insertion ; the middle fibres being horizontal, 
but as the origin of the muscle is very wide vertically, the 
upper portion descends in passing back, while the lower 
ascends. This strong and beautiful muscle has, from its pecu- 
liar situation, great influence over the jaw. The origin being 
more internal than the insertion, gives the muscle control 
over the condyle laterally, by which it is held firmly in the 
glenoid cavity, and the great strength of the muscle tends to 
keep the condyle from being driven back by falls, or blows 
upon the chin, which otherwise might occur easily, as the 
glenoid fossa is very shallow behind. 
A more prominent service of this muscle is when it brings 
the condyle forward either on its own side alone, as in tritura- 
tion, or wdth its fellow of the opposite side, as in cutting by 
the incisors. In these movements it acts in concert with other 
motors of the jaw. To consider it especially the triturating 
r\ 18 
muscle is no more correct than to suppose that when triturat- 
ing with a pestle in a mortar, the thumb and forefinger (which 
are further from the grinding surface) are the triturators rather 
than the fingers below. 
The external jpterygoid muscle controls the upper end of the 
jaw in concert with the temporal, wdiile the muscles attached 
to the body have especial control below, and by the concerted 
action of all, power and steadiness are secured in the compli- 
cated movements of the jaw, and of the lower teeth against 
the upper. The external pterygoid muscle has one office 
peculiar to itself, that of holding the condyle fixed on any 
part of the eminentia artieularis, to which it may be drawn 
in movements of the jaw. For this service the muscle is 
admirably fitted by the great width of its origin, which 
enables it to brace the condyle so firmly against the part 
above and in front of it, that the jaw is fixed, even when 
wfide open, as firmly as if the condyle were hinged in that 
position. 
The eminentia artieularis, the rounded' projection which 
forms the front of the glenoid fossa, is indispenable to this 
fixation of the condyle ; and as the condyle in coming forward 
mounts this eminence, the jaw, wdiile going dowrn in front, 
is also carried down bodily, by which the back teeth of the 
lower and upper jaws are widely separated. These advan- 
tages are gained while the centre, or rather the curve, upon 
which the jaw turns, is three-fifths down toward the angle. 
This may be tested by placing the wrist and hand around the 
back of the neck and applying the end of the middle finger 
to the back of the ramus, and finding the point upon wdiich it 
turns, wdiich is probably near the insertion of the internal 
lateral ligament. By this arrangement the jaw is opened 
promptly and widely, without interfering with the parts be- 
hind ; which would be injuriously pressed upon if the condyle 
remained still and the angle went back far enough to open the 
jaw wide. 
The external pterygoid is now7 to be spoken of in a service 
in which the action of the muscle is greater in range, frequency 
and importance than ill any other, and in which it has never 
before been recognized, that of opening the mouth. This mus- 19 
cle antagonizes the temporal, masseter and internal pterygoid 
muscles and is, especially by its lower head, the depressor of 
the lower jaw. It does this alone when necessary, without as- 
sistance from any other muscle. Its ability to do this may 
be proved by bracing the arm against the breast and applying 
the thumb firmly to the chin ; or better, by placing the jaw flat 
on the mantel, or any fixed support, then on opening the mouth 
the head will go backward, being drawn back by the exter- 
nal pterygoids whose insertions in the condyles are fixed by 
support of the jaw. 
The action of the external pterygoid muscles in opening the 
jaw is similar to their action when the jaw is brought forward 
to cut with the incisors, the difference in effect being produced 
by other muscles; the temporals probably act similarly in 
both movements. The masseters and internal pterygoids, 
however, must relax in the opening of the jaw instead of as- 
sisting to carry it forward as in cutting, while the digastrics, 
which relax in the forward movement of the jaw, undoubtedly 
assist to draw the chin back in gaping, vomiting, and in very 
wide opening of the mouth when voluntary. When not other- 
wise employed, it is probable that they always assist in open- 
ing the jaw, as they are admirably fitted for carrying the chin 
back, when the condyle is going forward. It is impossible 
that they could carry it back readily if the condyles were not 
at the same time pulled forward. For, when the chin points 
down very much (as in some persons even with the jaw closed), 
the digastric muscle forms but a slight angle at the loop, and 
decided action of it in such cases would tend to keep the con- 
dyle of the jaw back in the glenoid cavity from the start, or 
soon after, as the opening of the jaw would straighten the di- 
gastric so much that it would pull nearly in the direction of 
the ear and have but little power to carry the chin back. 
That the digastrics draw the condyles of the jaw back in the 
glenoid fossae when acting without decided action of the ex- 
ternal pterygoids, is demonstrated by exerting strong suction 
with the tongue against the roof of the mouth. In doing this 
the upper and lower teeth are held a little apart and the con- 
dyle of the jaw can be felt to move back, as the muscles tight- 
en, if the point of the little finger is pressed firmly into the 20 
ear so that its front rests against the auditory process. 
In cases where the anterior belly of the digastric ascends 
from, its loop to the chin, so that it has some vertical direction 
to favor it; and even supposing it were assisted by strong back 
fibres in the mylo-hyoid muscle, it is not possible that the con- 
dyle could release itself from the eminentia articularis ; and 
unless it did so and came forward, the jaw could open only to 
a trifling extent, for it would act as if hinged in the glenoid 
fossa. But with the external pterygoids drawing the condyles 
forward, the jaw opens readily, as upon a centre in the ramus, 
and the question arises as to what the jaw really turns upon. 
The masseter and internal pterygoid muscles hold the angle 
very much as if a sling were passed around it, while the stylo 
maxillary ligament is also inserted into the angle. It might 
therefore be thought that the jaw turned upon the angle if 
this part did not go back during the forward movement of 
the condyle, thus proving that the jaw turns on a portion of 
the ramus between the angle and the condyle. Under all the cir- 
cumstances the insertion of the internal lateral ligament around 
the inferior margin of the dental foramen marks the centre 
upon which the jaw opens with sufficient exactness. This 
ligament is a thin aponeurotic expansion, which descends per- 
pendicularly from the extremity of the spinous process of the 
sphenoid bone, and cannot properly be considered the special 
support of the jaw in its downward or upward movement. 
It is more likely that the pterygo-maxillary ligament also 
exerts an influence, and that all the ligaments and muscles 
inserted into this part of the bone are instrumental in deter- 
mining the centre upon which the jaw opens. 
The shape of the jaw, whether congenital, or modified by 
age, or accident, may also vary the location of the centre 
somewhat; but in every case the condyles must come forward 
to open the jaw effectually. This can be done only by the ex- 
ternal pterygoid muscles, whose special office is to move and 
also fix the condyles, and in connection with the temporal and 
other elevators, the jaw also. In this way only can the di- 
gastrics and associate muscles act efficiently in their most im- 
portant functions; otherwise they would be disturbed by undue 
movement of the jaw. Sometimes, however, the digastrics, 
as before stated, assist in moving and holding the jaw. 21 
When the external 'pterygoids throw the jaw wide open, the 
chin is much below the hyoid bone, but controlled by the an- 
terior bellies of the digastric muscles, the hyoid bone being 
fixed by the posterior bellies and by the oino-Jiyoid muscles, 
which can now be felt in action. Consequently the three ex- 
tremities of the jaw, the chin, the angle, and the condyle, 
have each diverging or converging muscular support. The 
digastrics go to the chin, the internal pterygoids go to the 
angles, and the external pterygoids to the condyles. This ar- 
rangement of the muscles in connection with the ligaments 
holds the jaw firm in all its complicated movements. 
It has been shown that the sterno-cleido-mastoid muscles do 
not rock the head, but that they give anterior support to the 
atlas, while the splenii muscles give posterior support, and 
that in combination they support it and the head laterally. 
This support is so complete that the head has a firm resting 
place, upon which it is held securely in all positions. The 
muscles which give form to the neck, and support and move 
the spine and head, are so arranged as to leave the front of the 
spine free for the tongue, larynx, and trachea, with their spe- 
cial muscles, &c., and for nerves, blood-vessels, and glands of 
prime necessity. 
These important parts are shut in and protected by the lower 
jaw, which gives attachment to muscles indispensable to those 
organs in the performance of their functions, while its own 
movements are in strict subordination to them. By this re- 
spiration is secured from interruption at all times, and occa- 
sionally made tributary to vocalization and articulation, and 
the vocal tube modified and moved with wonderful delicacy 
and energy in its participation in these functions, and in 
accordance with those of mastication and deglutition. 
The lower jaw is also the great lever by which the head is 
held upright. 
These explanations of the physiological action of the mus- 
cles which control and influence the lower jaw, prepare the 
way for the diagnosis of its fractures, upon which I purpose 
to write hereafter.