CHAPTER 7 

 THE MUSCULAR SYSTEM 



The muscular system of an active vertebrate makes up nearly half 

 the entire body-weight, in man slightly more than forty per cent. 



A muscle can do only one thing — contract. It cannot expand and, 

 having once contracted, must be pulled out to its resting length by one or 

 more antagonistic muscles. Each skeletal muscle consists of a fleshy part 

 or belly, each end of which is attached to a bone or cartilage, either directly 

 to the periosteum or indirectly by means of a tendon. The attachment 



TENDON- 



BELLY- -- 



'ARTERY 



VEIN 



NERVE 



TENDON 



Fig. i8i. — A diagram of the biceps muscle taken as a typical muscle, showing its 

 nervous and vascular relations. Each skeletal muscle is attached to a bone either 

 directly to the periosteum or indirectly — as in the case of the biceps — by means of 

 tendons. (Redrawn after Keith.) 



which moves most when the muscle contracts is its insertion; the other 

 is its origin. Each muscle is surrounded by a connective-tissue membrane 

 or perimysium, from which septa may grow into the muscle and divide 

 it into several muscle shps, each of which has a separate function. 



Muscles vary greatly in shape according to the arrangement of their 

 fibers and the way these are attached. Muscles may be segmented into 

 a series of similar units such as appear in the body muscles of fishes. 



igi 



