THE MUSCULAR SYSTEM 



49 



body this coagulation usually sets in at from four to six hours after 

 death, and continues for one, two, or three days. 



The erect position of the human body is maintained by the 

 combined influence of a large number of muscles acting at the same 

 time. The whole weight of the body rests on the arches of the 

 feet ; and the body may be supported in any position providing 

 its centre of gravity is situated vertically over any point in the 

 space enclosed by the feet. On account of the large number and 

 suppleness of the joints, the centre of gravity cannot be main- 

 tained in such a position as described above without the contraction 

 of certain muscles which give a degree of 

 rigidity to the body. (Fig. 54.) 



The muscles of the calf prevent the body 

 from falling forward, but the contraction of 

 these alone would pull the body backward, 

 as they act on the thigh-bone, just above and 

 behind the knee-joint. These muscles are 

 therefore opposed by another set in front of 

 the thigh, which connect the hip-bone above 

 with the bones of the leg below the knee, 

 .and which, by their contraction, pull the leg 

 straight. Then, again, these muscles tend to 

 pull the trunk of the body forward, but they 

 .are balanced by the powerful muscles of the 

 buttocks and 

 back. Further, 

 the contraction 

 of the muscles 

 of the back 

 of the neck 

 would pull the 

 head back ward, 

 were it not for 

 the contraction 

 of antagonistic 



muscles which connect the lower part of the face with the 

 sternum and collar-bone. 



When a person begins to walk, he first inclines the body for- 

 ward, then raises one foot, swings the leg forward one step, and 

 puts the foot to ground again. Now, just for a moment, the legs 

 form with the ground an isosceles triangle, and consequently the 

 trunk of the body is a little lower. But, before the foot reaches 

 the ground, the contraction of the calf of the other leg raises the 

 heel, and propels the body forward. The weight of the body is 

 thrown on the first foot, and the one behind swings forward and 



E 



Fig. 53. The Biceps Muscle. 



clavicle ; 2, scapula ; 3, humerus ; 4, radius ; 5, ulna ; 

 6, the biceps muscle. 



