THE MUSCULAR SYSTEM 277 



muscles of the head. Between the spinous and the transverse processes 

 of the neck vertebrae, are inter-spinales and intertransversarii muscles. 



On the ventral side of the throat, between the sternum and hyoid 

 bones, are the sternohyoid, stemo-th5rroid, thyrohyoid, and omohyoid 

 muscles, which pull the hyoid toward the sternum. The longus capitis, 

 longus cervicis, and the scaleni, anterior, middle, and posterior, are a 

 deeper group, ventral to the vertebrae, which bend the neck. 



Muscles of the Back. The dorsal muscles of the back extend from 

 the sacrum to the neck. Starting posteriorly as the sacrospinalis muscle, 

 the mass divides into a medial longissimus and a lateral ilio-costalis, both 

 of which as they extend into the neck region give off slips to all the ribs. 

 Closely connected with the ilio-costalis, is a more medial muscle, the 

 spinalis, which also extends along the backbone. In the groove between 

 the spinous processes of the vertebrae and the transverse processes, is a 

 group of muscles which have already been mentioned as the semi-spinalis 

 group in the neck. In the lumbar region this group is known as the 

 transversospinalis muscle. A deep subdivision of this group is the 

 multifidus. Between the vertebrae, connecting spinous and transverse 

 processes, are the inter spinales and intertransversarii muscles which 

 stiffen the spine. 



The prevertebral group of muscles of the trunk, immediately ventral 

 to the vertebral column, are less developed than those just mentioned, 

 so that the backbone is bent chiefly by other muscles not closely related 

 to the vertebrae. Prevertebral muscles are wanting in the thoracic 

 region. In the lumbar region, this group is represented by the major 

 and minor psoas. The psoas major is, however, more concerned with 

 bending the thigh than with flexing the back. The quadratus lumbonmi 

 bends the back sidewise. 



In the coccygeal region, the rudimentary and variable sacrococcygeus 

 muscles appear to be f unctionless but interest the morphologist as remnants 

 of the powerful caudal muscles of man's ancestors. 



Abdominal Muscles. The muscles of the lateral abdominal wall are 

 spHt into three layers, the external and internal obliqui, and the trans- 

 versus. The contraction of these muscles lowers the ribs, compresses 

 the abdomen, and by pushing the viscera against the diaphragm, expels 

 air from the lungs. Extending from the ribs to the pubic bone along 

 the ventral wall of the abdomen, is the segmented rectus abdominis, the 

 function of which is to lower the thorax and to flex the backbone. 

 Although this muscle is innervated by six or seven spinal nerves, it is 

 divided by inscriptiones tendineae into only four or five segments, which 

 are interpreted as remnants of the primitive metamerism of the trunk 

 musculature. The lateral halves of the rectus abdominis are separated 

 in the medial hne by a connective tissue linea alba. In the region where 



