THE SKELETON AND MUSCLES 



339 



or point of origin of the muscle; the other as the point of 

 insertion. As the muscle shortens, the bone upon which 

 the muscle is inserted moves. In the biceps, the shoulder 

 blade is the point of origin, the radius of the forearm, the 

 point of insertion. Fig. 174 shows other muscles of the 

 forearm, inserted by tendons. In some parts of the body, 

 as in the back, the muscles lie in sheets, one upon the other, 

 and are attached at different places on the bones so as to 

 render many movements possible (fig. 172). 



Certain sets of muscles are used for holding the body 

 erect; others are 

 used in moving the 

 whole body, as in 

 walking, running or 

 leaping; others for 

 moving parts of the 

 body as the arms, 

 the jaws, the eyelids, 

 eyes, etc. 



These muscles are 

 entirely enclosed in 

 a sheath of connec- 

 tive tissue called 

 perimysium. This 

 becomes extended 

 at the ends as tendons. From the inner side of this sheath 

 partitions pass inward, separating the groups of reddish 

 muscle fibers into larger and smaller bundles, and binding all 

 together into a distinct muscle. These partitions, or septa, 

 also afford support to the numerous blood-vessels and nerves 

 with which each muscle is supplied. Muscles of this type 

 have been called voluntary muscles because they are for 

 the most part under the control of the will. Such muscles 

 are also called striated muscles from their cross-striated ap- 

 pearance shown under the microscope. 



FIG. 173. Biceps muscle and bones of arm. 

 illustrating flexion of elbow joint when 

 muscle contracts. (After Martin.) 



