NECK. 



563 



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- 

 side them. The sterno-hyoid lies in its whole 

 length on the sterno-thyroid muscle and its 

 prolongation the thyro-hyoid, and these sepa- 

 rate 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 

 described 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 

 part of the cornu of the os hyoides, beneath 

 the omo-hyoid and sterno-hyoid, and superfi- 

 cially 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- 

 perior laryngeal nerve and artery as they pierce 

 the 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 

 crosses the coracoid notch, and from the ad- 



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 its 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 platvsma 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, 1824. 



2 o 2 



