THE ANIMAL BODY 



139 



the embryonic stages of higher forms the muscular layer is 

 composed .of segments known as MYOTOMES. But in the 

 adult stage of the latter this evidence of Vertebrate seg- 

 mentation largely disappears, since the muscular tissue for 

 the most part assumes the form of highly complex longi- 

 tudinal bands, extensions from which pass into the paired 

 appendages. 



A muscle consists of a very large number of muscle cells 

 bound together by connective tissue and permeated with 



FIG. 76. Vertical section of human skin, highly magnified, to show its com- 

 posite structure. Co, dermis; SM, Malpighian layer of epidermis; Se, outer 

 layer of epidermis; G, Gp, blood vessels; H, hair with sebaceous glands (D) ; 

 N, nerves; NP, sensory endings of nerves; SD, sweat glands with ducts opening 

 at SD* . (From Wiedersheim.) 



blood vessels and nerves. The muscle cells themselves have 

 in a highly developed and specialized form a primary attri- 

 bute of all protoplasm, contractility, which they exhibit by 

 shortening and broadening when stimulated by impulses 

 reaching them through the nervous system. Muscles, such 

 as those attached to the bones, in which contraction can be 

 brought about at will, are termed VOLUNTARY muscles, while 

 those which cause most of the movements of the viscera are 

 known as INVOLUNTARY muscles. (Fig. 7, E, F.) 



