86 ANATOMY AND PHYSIOLOGY 



the spine in the erect position; also they assist to move it in 

 various directions. 



The erector spinae is the name given to this large group, 

 which is bound down in its place by a thin layer of fibrous tissue 

 called the vertebral aponeurosis. This muscle group extends from 

 the skull to the lower part of the sacrum (Fig. 74). 



The action is most easily seen in the lumbar and dorsal regions, 

 where it is not deeply covered with other muscles. 



Nerves. Posterior spinal. 



The latissimus dorsi (broadest of the back, Fig. 75). This 

 muscle covers most of the erector spinae and a great portion of 

 the back of the trunk. 



Origin. The spinous processes, from the sixth thoracic down 

 to the end of the column. (Also the crest of the ilium and a few 

 fibers from the inferior angle of the scapula.) Insertion. The 

 crest of the lesser tubercle of the humerus. 



Action. Principally to pull the arm backward and keep the 

 scapula or shoulder-blade close to the chest; brought prominently 

 into use in rowing a boat or when the body is suspended by the 

 hands and an effort is made to draw it up. 



Nerves. Posterior spinal and long subscapular. . 



MUSCLES OF THE BACK OF THE NECK 



These muscles move the head and neck. Only the most 

 important are here described. 



The splenius. This muscle is in two portions, the splenius of the head 

 (capitis) and the splenius of the neck (cervicis). 



Origin. The spinous processes of the last cervical and first six thoracic 

 vertebrae. Insertion. Partly upon occipital and mastoid bones (splenius 

 capitis) and partly upon the transverse processes of the upper vertebrae 

 (splenitis cervicis). 



Action. The muscle of one side alone will rotate the head, twisting the 

 neck. The muscles of both sides acting together simply pull the head back- 

 ward or extend it and the neck. 



Nerves. Posterior cervical. 



The trapezius covers the other muscles of the back of the 

 neck, and also the upper portion of the latissimus dorsi. It is one 

 of the largest muscles in the body (Fig. 75). The two muscles, 

 right and left together, make a large diamond-shaped sheet. 



