564 
internally joins a similar process from the op- 
posite side to form with it a tendinous expan- 
sion, (often assisted by a few fleshy fibres from 
the anterior belly of the digastric,) which reaches 
from the os hyoides as far as the jaw, and con- 
tributes to support the floor of the mouth. 
The relations of this muscle are complicated 
and important: the convexity of its curve is 
the upper limit of the anterior triangle of the 
neck ; its concavity bounds a space, the area 
of which extends within the jaw to the myloid 
ridge, containing various parts, and named 
from the muscle the digastric space; its pos- 
terior belly crosses the external and internal 
carotid, the facial, lingual, and occipital branches 
of the former, the internal jugular vein, the 
three divisions of the eighth, the ninth, and the 
sympathetic nerve, the side of the pharynx, the 
trachelo-mastoid and styloid muscles, and the 
hyo-glossus. The sternu-mastoid and splenius 
cover its origin; the portio dura emerges at its 
anterior edge, along which the posterior aural 
artery runs, and round which the posterior part 
of the parotid gland is folded. Its anterior 
belly and tendon support the submaxillary 
gland, are covered by fascia and platysma, and 
correspond to the mylo-hyoid muscle, which 
is covered and strengthened by the aponeurotic 
expansion derived from the digastric. 
he action of this muscle varies according to 
the fixity of the jaw: when the mouth is firmly 
closed, the contraction of the two bellies will 
draw the hyoid bone vertically upward, and 
communicate to the pharynx the movement 
of elevation, which adapts it for receiving the 
masticated food. A firm closure of the jaw, a 
contraction of the digastric muscles, and conse- 
quent shortening of the pharynx, (indicated by 
rising of the pomum Adami,) are well known 
acts in the process of deglutition. When the 
hyoid bone is fixed by its depressors (and 
perhaps in some degree radiniea by the joint 
actions of the mec belly of the digastric 
and of the omo-hyoid), the anterior belly, both 
sively as a reflected cord, and actively, 
in virtue of its muscular fibres, depresses the 
lower jaw and opens the mouth. Simulta- 
neously, too, with its act of raising the pharynx, 
this muscle must tighten, by its posterior belly, 
the mesial aponeurotic expansion, which joins 
it to its fellow; and, by so doing, must assist 
the mylo-hyoid in raising the floor and reducing 
the capacity of the mouth. It fulfils, there- 
fore, important uses in the mechanism of de- 
glutition. 
The stylo-hyvid muscle is an accessory to the 
posterior belly of the digastric, and arises from 
the outer surface of the styloid process, about 
midway from its base, by a small round tendon, 
which soon swells into an elongated body. 
This lies along the posterior belly of the digas- 
tric, parallel to its anterior edge, and, when it 
reaches the os hyoides, is inserted into the outer 
surface of that bone, at the union of its body 
and cornu, by short aponeurotic fibres. It 
usually divides, just previously to its insertion, 
to give passage to the tendon of the digastric. 
The portio dura of the seventh pair emerges 
between its origin and that of the digastric: in 
NECK. 
‘rated from the mylo-hyoid muscle by 
other respects its relations so entirely agree with 
those of the descending belly of that muscle, as 
do likewise its uses, that no particular descrip- 
tion of these is necessary. ek 
The mylo-hyoid muscles are so mutually de 
pendent that they might almost be de: 
as a single muscle. They arise on either sid 
from the oblique or myloid ridge on the buc 
surface of the lower jaw in its whole exten 
i. e. from opposite the last molar tooth to the 
neighbourhood of the symphysis. The flesh 
fibres, that succeed = rt ve SIS | 
origin, proceed lelly toward medi 
line, ee. are eae a raphe, which reache 
from the symphysis of the jaw to the body 
the hyoid bone, and likewise into the upper 
border of the body of that bone. The an' 
fibres are short ; those which succeed progre 
sively increase in length, and the posteriol 
which are fixed to the hyoid bone, are of all th 
longest. Each muscle is, therefore, triangular 
having an outer edge by which it rises fi 
the jaw, an inner edge of union with its fe 
and a posterior edge, which is seen to ext 
in the digastric space, from the posterior € 
mity of the myloid ridge to the upper edg 
the body of the hyoid bone, close to its cor 
The under surface of the muscle corr ¢ 
the submaxillary gland and to the insertio 
the digastric; its upper surface sustains 
tongue and floor of the mouth,—from t 
cous membrane of which it is separated 
Wharton’s duct, the sub-lingual gland, and 
tatory nerve; it also corresponds to the 
glossus, genio-hyoideus, and genio-hyo-gle 
and to the termination of the ingeay aur 
and nerve. The duct of the sub-max 
gland winds round its posterior edge, in pi 
ceeding to open beside the frenum lin 
The habitual state of this muscle is om 
which it is rendered, with its fellow, 
downward by pressure of the superine 
parts; and so its surfaces cannot 
said to face upward and downward, 
a modification of these directions r 
inward and outward. Thus the twom 
furnish a concave floor to the mouth, ane 
only in their contraction, which acco 
diminishes the cavity, that this becomes! 
horizontal. Their action, especially w 
sisted by other muscles, is to propel t 
ticated food by lessening the capacity 
mouth. an 
The hyo-glossus is a thin quadril: 
of parallel muscular fibres, a 1e 
ments which its name indicates. It ms 
the entire length of the great cornu ‘ 
joining part of the body of the os hyoi 
their upper surface, and ascends to be: 
into the side of the tongue. From bet 
anterior thicker edge the lingual artery 
its posterior thin border receives the 
of the stylo-glossus; its deep s 
fer to the genio-hyo-glossus 
rom the former of which it is par 
4 
baal 
by the lingual artery ; its e 
gual and gustatory nerves and duet of # 
maxillary gland. my 
