; 
L 
from their fellows on the opposite side only by 
the mesial raphe of the cervical fascia,—cover- 
ing the trachea, thyroid body, with a portion of 
the larynx, and overlapping the sheath of the 
carotid vessels. They are isolated from each 
other, and from the other muscles of their neigh- 
bourhood, by processes of the cervical aponeu- 
rosis. The sterno-hyoid arises just within the 
thorax from the deep surface of the manubrium 
Sterni, from the cartilage of the first rib, and 
from the ligament of the sterno-clavicular joint, 
and is separated from that of the opposite side 
by nearly the whole breadth of the sternum. 
As it ascends, it more nearly approaches its 
_ fellow, and the two are inserted side by side into 
the under surface of the body of the os hyoides, 
in close connexion, by their outer edges, with 
the omo-hyoid muscles, which are inserted be- 
sidethem. The sterno-hyoid lies in its whole 
length on the sterno-thyroid muscle and its 
prolongation the thyro-hyoid, and these sepa- 
fate it from immediate contact with the impor- 
tant organs to which it is related. 
The sterno-thyroid is broader and rises lower 
‘within the chest,—from the cartilage of the 
‘second rib, and from the adjoining surface of 
_ the sternum, on which it extends almost to the 
median line: its fibres ascend nearly vertically, 
‘and terminate at an oblique fibrous arch on the 
_ ala of the thyroid cartilage, and at the tubercles, 
to which this arch is attached ; hence a muscle 
_ of similar volume is prolonged, (which may be 
lescribed as rising from the oblique cord and 
from its points of attachment, but which, in 
_ direction, size, and form, accurately continues 
_ the sterno-thyroid,) and, after a course of an 
inch and a half, is inserted into the body and 
a of the cornu of the os hyoides, beneath 
the omo-hyoid and sterno-hyoid, and superfi- 
_ @ially to the thyro-hyoid membrane. To this is 
"given the name of thyro-hyoid. 
__ The sterno-thyroid and thyro-hyoid are co- 
 vered throughout by the sterno-hyoid and in 
part by the sterno-mastoid and omo-hyoid 
‘muscles. The sterno-thyroid corresponds by 
its inner edge to the inferior thyroid vein,—by 
‘its outer edge receives the terminal branch of 
the descendens noni, by its deep surface covers 
the thyroid body and many of its vessels, the 
trachea and part of the larynx, and the sheath 
of the carotid vessels: by its origin it enters 
into the mediastinum, covers the great arterial 
trunks springing from the arch of the aorta and 
‘the brachio-cephalic veins. From these parts 
it is separated by the remains of the thymus 
gland. The thyro-hyoid muscle covers the su- 
ng laryngeal nerve and artery as they pierce 
wall of the larynx. These muscles are 
fleshy in their whole extent, with exception of 
the short tendinous fibres, by which they 
have their origin and insertion: the sterno- 
thyroid has frequently a transverse tendinous 
intersection in some part of its course. 
The omo-hyoid is a slender but long bi-ventral 
muscle, obliquely extending from the superior 
costa of the scapula to the os hyoides. It 
arises by short tendinous fibres at the root of 
the coracoid process, from the ligament which 
_ Grosses the coracoid notch, and from the ad- 
NECK. 
563 
joining part of the costa, directs itself with a 
slight ascent towards the median line, and, in 
emerging from behind the clavicle, frequently 
derives a few fibres from i\; posterior edge. It 
contracts to a flattened tendon as it passes be- 
neath the sterno-mastoid, and abruptly changes 
its direction from a nearly horizontal to a ver- 
tical course, by undergoing a trochlear re- 
flexion in a loop of the cervical fascia,—and, 
again becoming fleshy, ascends beside and 
parallel to the outer edge of the sterno-hyoid, 
to which it is closely united,—to be inserted 
into the lower border of the hyoid bone at the 
junction of its body and cornu. The very im- 
portant relations of this muscle will be more 
fully given in the detailed surgical anatomy of 
the region. It may for the present suffice to 
say, that, in crossing the direction of the sterno- 
mastoid muscle, it furnishes the subdividing 
line of the great triangles of the neck; that its 
posterior belly lies parallel to and just above 
the subclavian artery and brachial plexus, is 
covered by the platysma and partly by the 
trapezius, clavicle and subclavius, and crosses 
the scaleni and phrenic nerve: that its looped 
tendon is covered by the sterno-mastoid, and 
lies on the sheath of the carotid vessels, across 
which its anterior belly continues obliquely to 
run. 
The two omo-hyoid muscles acting in con- 
cert are capable of depressing the os hyoides ; 
but their chief action is of a different nature. 
Being contained in their whole bent course 
within a sheath of cervical fascia, they affect 
this membrane by their contraction, tensely 
Spanning it across the median line in a space 
which extends from the hyoid bone to its 
clavicular attachment. This appears to be one 
of the consensual movements in the act of de- 
glutition, designed to give, during that act, 
additional efficacy to the protection against at- 
mospheric pressure, which Burns has shown to 
be an important function of the fascia of the 
neck.* 
The digastric muscle is likewise, as its name 
imports, double-bellied; it passes from the 
mastoid process of the temporal bone to the 
symphysis of the jaw, but is looped down in 
its course to the side of the os hyoides. Its 
temporal attachment is to the groove, which is 
named from it, on the inner surface of the 
mastoid process: a large fleshy belly proceeds 
from this origin downward and forward, con- 
tracts to a round tendon, which usually pierces 
the stylo-hyoid muscle, traverses an aponeu- 
rotic ring lined by synovial membrane, which 
strongly binds it to the hyoid bone, near its lesser 
cornu, and is then reflected upward, expanding 
again to a strong muscular belly, which fixes 
itself by short aponeurotic fibres into the lower 
border of the jaw, at an oval depression be- 
side the symphysis. Its tendon, just after pass- 
ing through the fibrous pulley that maintains 
its curve, gives off a fascial process toward 
the median line: this attaches itself strongly 
along the upper edge of the hyoid bone, and 
* Surg. Anat. of Head and Neck, p. 36. Glas- 
gow, 1 
202 
