MAN, A MAMMAL 



323 



ment in a contraction is called 

 the insertion; the part that moves 

 least is the origin. 



Microscopic Structure of Volun- 

 tary Muscle. — With a sharp pair of 

 scissors cut through a muscle at right 

 angles to the long axis ; examina- 

 tion will show that it is composed of 

 a number of bundles of fibers. These 

 fibers are held together by a sheath 

 of connective tissue. Each of these 

 bundles may be separated into smaller 

 ones. If we continue this so as to 

 separate into the smallest possible bits 

 that can be seen with the naked eye, 

 and then examine such a tiny portion 

 under the compound microscope, it 

 will present somewhat the appearance 

 shown in the Figure. The muscle is 

 seen to be made up of a number of tiny 

 threads which lie side by side, held 

 together by the sheath. Muscles, 

 then, are bundles of long fibers. In 

 man, muscles which are under the 

 control of the will have a striated 

 appearance, while those which 



A bit of voluntary muscle fiber, shoN^ng 

 the cros-i striations as seen under the 

 microscope. (Highly magnified.) 



are involuntary are unstriated. Both 



kinds are supplied with nerves, which control them (see Figures). 



The delicate endings of nerves in vol- 

 untary muscle. (Highly magnified.) 



Muscle Tissue and its Uses. — 

 Muscles evidently form a large 

 part of the body, in man, nearly 

 half the body weight being muscle. 

 Nearly every muscle in the human 

 body is attached to a bone either 

 at one or at both ends. Move- 

 ment is performed by means of the 

 muscles, leverage being obtained 

 by means of their attachment to 

 the bones. Movement is, indeed, 

 the chief function of muscles. In 

 the human body there are over 



