350 THE MUSCULATURE 



than normal or may be missing. The attachments to the scapula show considerable variations. 

 Occasionally the cervical and thoracic portions are separate, a condition normal in many 

 mammals. Ventrally the trapezius may become continuous with the sterno-cleido-mastoid 

 in the neck, or send a fasciculus to it or to the sternum. Aberrant fasciculi are not infrequent. 

 Rarely a transverse tendinous inscription is found in the cervical or in the thoracic portion of 

 the muscle. Sometimes a fasciculus is sent into the deltoid. The , innervation of eithec-iJie 

 sterno-cleido-mastoid or the trapezius may be by cervical nerves only. The omo-cervicalis 

 (levator claviculce) is a fasciculus frequent in the lower mammals, but rarely found in man. 

 It usually extends from the acromial end of the clavicle to the atlas and axis, but may extend 

 to more distal cervical vertebrae. It is innervated by a ramus from the cervical branches to the 

 trapezius. The supra-davicularis proprius is a muscle rarely found. It extends on the cranial 

 surface of the clavicle from the sternal to the acromial end and is innervated by the third cervical 

 nerve. It is said to make tense the superficial layer of the cervical fascia. 



A bursa is often found between the base of the spine of the scapula and the tendon of inser- 

 tion of the thoracic portion of the trapezius. Another bursa is also frequently found between 

 the insertion of the transverse portion and the supraspinous fascia. 



6. INFRAHYOID MUSCULATURE 



(Figs. 348 and 351) 



The four infrahyoid muscles constitute a well-defined group of muscles which 

 depress the hyoid bone, the larynx, and the associated structures. They lie 

 beneath the sterno-cleido-mastoid muscle and the external cervical fascia. Two 

 strata may be recognised. In the superficial stratum are comprised the omo- 

 hyoid, a narrow, ribbon-like digastric muscle which arises from the superior 

 margin of the scapula and is inserted into the hyoid bone; and the thin, quad- 

 rangular sterno-hyoid, which arises from the superior margin of the sternum and 

 the medial end of the clavicle and is inserted into the hyoid bone. Between 

 these two muscles is an aponeurotic membrane which constitutes the main part 

 of the middle layer of the cervical fascia, and represents possibly a retrograde 

 portion of a single muscle, of which the two above named are but the ventral and 

 dorsal margins. Beneath this superficial musculature the thin, quadrangular 

 thyreo-hyoid descends from the hyoid bone to the thyreoid cartilage, and the 

 ribbon-like stemo -thyreoid arises from the dorsal surface of the manubrium 

 and is inserted into the thyreoid cartilage. 



All these muscles are supplied by branches from the ansa hypoglossi. The 

 nerve-fibres arise from the first three cervical nerves. 



The muscles of this group are derived from the ventral portions of the ventro-lateral divi- 

 sions of the first three cervical myotomes, and correspond with the rectus abdominis muscle, 

 which is derived from the ventral portions of the eighth to the twelfth thoracic myotomes. 

 This musculature is characterised by metameric segmentation, which may be more or less ob- 

 scured, and by a general longitudinal direction taken by the component fibre-bundles. The 

 course of the fibres in the omo-hyoid may be looked upon as a secondary condition due to the 

 shifting laterally of the distal attachment of the muscle. Musculature of this nature is not 

 derived from the lower cervical and upper thoracic myotomes in man, but in some of the lower 

 vertebrates it forms a continuous ventral band. Even in man occasional traces of this ventral 

 musculature may, however, be seen as muscular and aponeurotic slips on the upper part of the 

 thoracic wall, above the ribs and the aponeurosis of the external intercostal muscles. 



FASCIA 



(Figs. 351 and 357) 



The middle cervical fascia is composed of two lamina). Of these, the superficial, which 

 enshcaths the sterno-hyoid and omo-hyoid muscles and fills in the intervening area, is much the 

 stronger and better differentiated. The more delicate deep lamina ensheaths the thyreo-hyoid 

 and .stcrno-thyreoid muscles, and laterally extends out to become fused with the superficial 

 lamina. It is also more or less closely bound to the sheath which covers the internal jugular 

 vein, carotid artery, and vagus nerve. 



The middle fcrvicul fascia is attached above to the hyoid bone. Beyond the lateral edge 

 of the omo-hyoid it becomes fused with the deep lamina of the external layer of the cervical 

 fascia, beneath the sterno-cleido-mastoid. Posterior to this muscle it usually terminates along 

 the cranial margin of the omo-hyoid in the areolar tissue of the neck. Its distal attachment 

 takes place into the donsal surface of the upper margin of the sternum, and from here a process 

 is sent over the left innominate vein to the pericardium. Lateral to the sternum the fascia is 

 atta(;hed for .some distance to the inner margin of the clavicle, and gives rise to processes, one of 

 which extends to the fascia of the Kul)clavius muscle, while the others pass on each side of the 

 subclavian vein to the first rib. Still more laterally the fascia is fused along the lower margin 

 of the scapular belly of the omo-hyoid to the underlying dense, fatty areolar tissue. 



