190 HISTORY OF THE HUMAN BODY 



The muscles of the second type are in general under the 

 control of the animal's will, and are thus termed voluntary, 

 although there are regions, as along the pharynx and oesopha- 

 gus, where genuine striated muscle may become involuntary 

 and depend for its action upon external stimuli. 



The striated muscles, which form the subject of this chap- 

 ter, fall into three anatomical divisions corresponding to those 

 of the skeleton, the axial muscles, the appendicular muscles, 

 and the visceral muscles. In the embryo the appendicular 

 muscles are derived directly from the axial, both arising from 

 the dorsal portion of the mesodermic somites, the epimeres; 

 while the visceral muscles, limited to the anterior part of the 

 body, arise from the ventral portion, the hypomeres, the ele- 

 ment which in the remainder of the body develops into the 

 pleuro-peritoneum, and furnishes no voluntary muscles. This 

 difference in origin sharply divides the voluntary musculature 

 into two fundamental groups, (i) the parietal, arising from 

 the epimeres and including the axial and appendicular mus- 

 culatures, and (2) the visceral, arising from the hypomeres, 

 and limited to the anterior part of the body. 



Aside from these three primary divisions of striated mus- 

 cles, there occur in almost all vertebrates certain superficial 

 muscular elements, usually in the form of subcutaneous layers 

 and intimately associated with the corium. These have often 

 been treated as a distinct group of muscles, but recent investi- 

 gation has placed it beyond doubt that we have here to do with 

 muscular elements which have developed independently in dif- 

 ferent animals in response to certain physiological needs and 

 have been derived from the most convenient subjacent skeletal 

 muscles, whether 'parietal or visceral. However, in spite of 

 their secondary origin from other muscular groups, they have 

 differentiated so far structurally that it is far more convenient 

 to treat them as a distinct group, the integumental muscles, 

 rather than to consider them with the various muscles from 

 which they were originally derived. The order of treatment, 

 therefore, of the muscles, as described in this chapter, will 

 follow the plan just outlined: the first to be considered will 



