THE NECK 



merotrapezoid on the other — work in either direction from the anterior 

 hmb, so that in such motions the arm acts as a sort of raphe between 

 these two groups of muscles. These muscles concerned with lateral 

 neck movements are very robust and diverge from the head to the sides 

 of the broadened thoracic region in a manner to give them very powerful 

 leverage. They must certainly have an extraordinary amount of normal 

 tone, so that when those of one side contract the muscles upon the op- 

 posite side will not relax to a greater extent than is proper. And yet, 

 when the animal wishes to stretch forth its head, the tone is removed 

 and the muscles are relaxed to an abnormal degree. The same may be 

 said of the muscles chiefly concerned with movement of the head in 

 the sagittal plane — the anterior rhomboid and semispinalis for raising, 

 and sternohyoid and tracheovertebral muscles for lowering it. They 

 are powerful and well situated for performing the work which they 

 have to do, which evidently consists chiefly in strong but short depression 

 or elevation of the head to facilitate steering. 



As before mentioned it seems certain that because the head of the seal 

 is rather small, instead of relatively large, for the size of the body, the 

 lateral movements of the neck which accompany the act of swimming 

 are distributed over the entire cervical series of vertebrae, and they ac- 

 cordingly would not be expected to depart widely from the usual type. 

 Really the only noteworthy detail is the fact that the spinous processes 

 are reduced and very much smaller than in the sea-lion, which seems an 

 indication that active strength (as contrasted with passive strength) in 

 elevating the head is not of great importance to the animal. 



The only ways in which the principle of aquatic locomotion of whales 

 differs from that in seals is that in the former the flattened tail consti- 

 tutes the primary organ for propulsion instead of the hind feet, and that 

 its plane of movement is vertical instead of horizontal. This means 

 that in the Cetacea the dorsal and ventral neck muscles are required to 

 perfom those acts during swimming which constitute the chief function 

 of the lateral neck muscles of the seal. But the lateral neck muscles are 

 attached to the thorax or arm and belong in entirely diiferent groups from 

 those which swing the tail laterally, while the dorsal neck muscles of chief 

 use are an integral part of the spinal musculature of the entire animal. 

 This permits the fundamental differences that in the seal the center of 

 swimming motion must be in the anterior thorax, from which the neck 

 muscles arise, while in the whale there is no muscular inhibition to its 

 shifting either forward or backward, according to requirements intro- 

 duced by external body form. 



[ 145} 



