MUSCULAR SYSTEM. 113 



use, that is, in post-embryonic time. Certain muscles may disappear in the 

 course of development, and changes of position may also take place. 



Wherever a marked friction occurs, ossifications (sesamoids) may become 

 developed in the course of a muscle or tendon. The muscle thus gains an 

 extra point of attachment, and a single-jointed lever is converted into a 

 double-jointed one. 



The higher one passes in the animal scale, the more numerous 

 do the differentiations of the muscular system become, and the 

 more varied its relations to the skeleton. One portion, the dermal 

 musculature, which is sometimes not largely developed, always 

 shows an independence as regards the rest of the muscular sys- 

 tem, though this independence is as a rule acquired secondarily. 

 Developed to a very slight extent in Fishes and Amphibia, the 

 dermal musculature is of great importance in Reptiles and Birds on 

 account of its relations to the scutes, scales, and feather?. It reaches 

 its greatest development amongst Mammals, where it may extend 

 over the back, head, neck, and flanks (Echidna, Dasypus, Pinni- 

 pedia, Erinaceus, &c.). In Man, only a slight rudiment is found of 

 it in the shape of the platysma myoides, which extends over 

 the neck and part of the breast and face. 



MUSCULATURE OF THE SKELETON. 



MUSCLES OF THE TRUNK. 



Under this head are included all the muscles of the body 

 which remain after the removal of those connected with the limbs. 

 They arise from the muscle-plates of the embryo, that is, from the 

 outer parts of the mesoblastic somites, 1 and, particularly in higher 

 types, may be separated into various groups, viz., a cranial and 

 visceral, a dorsal and ventral. 



Fishes and Amphibia. In Fishes and the lower Amphibia 

 the dorsal and ventral groups of muscles form a uniform mass, 

 which is spoken of as the lateral body -muscle (M. lateralis). 

 On each side of the body this consists of two portions, a dorsal 

 and a ventral, which meet together laterally, as well as in the 

 mid-dorsal and ventral lines (Fig. 97, D, F), and which are made up 

 of a great number of metamerically arranged portions (myotomes 

 or myocommata), separated from one another by connective- 

 tissue septa (Fig. 97, M, M). Along the latter, ribs may be 

 developed, and thus a much greater degree of firmness is attained. 



This metameric arrangement of the musculature of the trunk, 

 which has such an important relation to the spinal nerves as well 



1 The development of the sub- vertebral, cutaneous, and diaphragmatic muscles, 

 requires further investigation. 



I 



