GROUPS OF MUSCLES 327 



the vagus supplies all the muscles but the crico-thyreoid. This is supplied by the superior 

 laryngeal branch of the vagus. 



Metamerism. — The muscles thus far considered are essentially visceral muscles, although 

 all are composed of striated muscle cells and all are more or less directly under the control of 

 the will. From the morphological standpoint the muscles of the orbit, the tensor tympani, 

 the muscles of mastication, the hyo-mandibular muscles and the muscles of the tongue have 

 been grouped with the ordinary voluntary skeletal muscles while the facialis musculature, the 

 stapedius, the hyo-temporal muscles and the muscles of the soft palate, pharynx and larynx 

 are looked upon as of a more purely visceral origin. A primitive characteristic of the voluntary 

 skeletal muscles is metameric segmentation. This is maintained through life in the trunk 

 musculature of fishes and of tailed amphibia and is passed through as a temporary stage in 

 all the higher vertebrates. The embryonic segmented muscles are called myotomes (see fig. 

 340). In some regions the metamerism is retained throughout life even in the higher forms, 

 as, for instance, in the intercostal muscles and the intertransverse muscles. But for the most 

 part the primitive metamerism is so lost during the differentiation of the definitive trunk 

 musculature that only traces of it remain here and there as, for instance, in the segments of the 

 rectus abdominis muscle. In the lower forms the myotomes give rise during embryonic 

 development to material utilized in the formation of the limb musculature, but even in the 

 fishes all traces of trunk metamerism are lost in the differentiated limb musculature and in the 

 higher forms, as in man, the limb musculature appears to differentiate directly from the un- 

 segmented tissue in the limb-buds. In the head the musculature is differentiated directly, as 

 in the limbs, without undergoing a prehminary metameric or myotomic stage. Attempts 

 have been made to show that in primitive forms the cranial voluntary skeletal musculature, in 

 the narrower morphological sense mentioned above, passes through a metameric stage com- 

 parable with the myotomic metamerism of the trunk. This attempt has been partially success- 

 ful as regards the development of the muscles of the eye in some of the lower forms. There 

 is also good evidence that the spinal region of the skull and associated structures represent a 

 part of the metameric trunk fused with a more primitive head so that the musculature of the 

 tongue and the hyo-mandibular muscles belongs morphologically with the primitively metameric 

 trunk musculature. The rest of the cranial musculature gives little evidence of a primitive 

 metameric segmentation and hence is probably to be classed morphologically with the unseg- 

 mented visceral musculature. 



Of the muscles of the neck, the most superficial, the platysma (fig. 341), is a subcutaneous 

 muscle belonging to the facialis group of the head from which it grows down during embryonic 

 development. It is supphed by the seventh cranial (facial) nerve. It extends from the corner 

 of the mouth and the side of the mandible over the clavicle. It depresses the corner of the 

 mouth, wrinkles up the skin of the neck and aids the circulation by relieving pressure on the 

 underlying veins. 



Beneath the platysma there lies a layer composed of two flat muscles (fig. 344) which 

 extend from the base of the skull behind the ear to the shoulder girdle. One of these muscles, 

 the sterno-cleido-mastoid, arises in front from the sternum and clavicle and is inserted into the 

 mastoid process of the temporal bone and the skull behind this. The other, the trapezius, arises 

 from the base of the skull, and from the ligamentum nuchse and vertebral spines of the cervical 

 and thoracic regions, and is inserted into the spine of the scapula, the acromion and the lateral 

 third of the clavicle. These two muscles constitute the superficial shoulder-girdle musculature. 

 They extend the head, bend it toward the same side and rotate it toward the opposite side. 

 The sterno-cleido-mastoid and the upper part of the trapezius raise the shoulder girdle and 

 thorax and hence are of use in forced inspiration. The trapezius draws the scapula toward the 

 spine and rotates the inferior angle of the scapula lateralward. The lower part of the trapezius 

 acting alone draws the scapula downward and dorsalward while rotating the inferior angle 

 lateralward. The trapezius is therefore used when the arm is raised high or carried backward. 

 The two muscles of this group are innervated partly by the spinal accessory, and partly by the 

 ventral divisions of the second, third and fourth cranial nerves. They represent in part 

 musculature which in the lower vertebrates is associated with the visceral musculature of the 

 gills (hence the innervation by the spinal accessory, a derivative of the vagus nerve) and in 

 part metameric musculature of the second, third, and fourth cervical segments. During 

 embryonic development this musculature therefore spreads out widely from its origin, the upper 

 cervical region. The lower part of the trapezius varies greatly in the extent of its development 

 caudalward. It may reach only half way down the thoracic region or it may extend into the 

 lumbar region. The deeper musculature of the neck is derived from the cervical myotomes. 



The primitive segmental musculature of the neck, like that of the whole trunk, becomes 

 divided at an early embryonic stage into two divisions, a dorsal, supplied by the dorsal divisions 

 of the spinal nerves, and a ventro-lateral supplied by the ventral divisions. The trapezius, 

 although it covers the intrinsic dorsal musculature of the cervical region, insofar as it is of 

 cervical origin, belongs to the ventro-lateral musculature and is derived, apparently, from the 

 first three cervical myotomes. There is also a deeper layer of muscles attached to the shoulder 

 girdle which arise from the ventro-lateral divisions of the lower five or six cervical myotornes 

 but which, with one exception, the levator scapulce (fig. 353), wander over the thorax during 

 embryonic development. This group is described below as the deep shoulder-girdle muscula- 

 ture. The rest of the muscles derived from the ventro-lateral divisions of the cervical myo- 

 tomes are divisible into three groups, the infra-hyoid, the scalene and the prevertebral. 



The infra-hyoid group lies at the front of the neck, superficial to the larynx and trachea 

 (fig. 348), and is composed of four flat muscles, the sterno-hyoid, sterno-thyreoid, thyreo-hyoid 

 and omo-hyoid (scapulo-hyoid), the names of which indicate the origin and insertion. The 

 chief function of this group of muscles is to depress the hyoid bone, the larynx and the as- 

 sociated structiu-es. When the supra-hyoid group of muscles contracts at the same time, the 

 infra-hyoid muscles help to depress the lower jaw, or if this in turn is fixed by the cranio-man- 

 dibular group, to flex the head. The muscles of this group are derived from the ventral portions 



