60 THE HUMAN BODY. 



part in effort they must have a point of support or fulcrum, 

 directly or indirectly upon the skeleton of the trunk, that is 

 on the spinal column and the bones of the thorax. An effort 

 is always preceded by an inspiration which dilates the thorax, 

 thus rendering the bones of which it is composed immovable 

 by the contraction of the inspiratory muscles, and furnishing 

 a fixed point to the muscles attached to these bones. Thus, 

 one after another, the greater part of the muscles of the 

 system joins in a movement of which sometimes the arm or 

 the hand only is the immediate instrument. The proof of 

 this is the impossibility of making a movement distinct from 

 that which is the immediate object of the effort, without this 

 effort ceasing or diminishing. When it is at its highest point, 

 respiration is suspended, the glottis closes or remains slightly 

 open, according to the nature and the degree of intensity in 

 the effort ; the inspired air distends the lungs, and if a part 

 escapes it is too small to lessen the expansion of the chest; 

 the abdominal viscera are compressed from above by the 

 diaphragm in front, and laterally by the muscles of the abdo- 

 men. During certain efforts the air is expelled slowly through 

 the glottis, and when the movement terminates suddenly 

 with redoubled force, the expiration is accomplished rapidly 

 and sometimes in the form of a cry. The sailor who hauls 

 in a rope, or the baker as he with difficulty raises the dough 

 to throw it back into the kneading-trough, accompanies the 

 movements which he executes with a cry of which the rhythm 

 expresses the different periods of the effort. 



Locomotion. Man moves over the surface of the ground 

 by three principal methods of progression walking, running, 

 and leaping ; but the point of departure is always the vertical 

 position. In this attitude, which characterizes the human 

 race, the equilibrium of the head upon the vertebral column, 

 and that of the trunk upon the coxo-femoral articulations, 

 and of the thighs on the legs, is independent of all muscular 

 contraction, the ligaments being sufficient to insure it. And 

 farther, the muscles of the neck, of the trunk, and of the 

 thigh maintain the rigidity of the spinal column, oppose or 

 prevent the flexion of the knee, and restore the equilibrium 

 when it is compromised, while the muscles of the leg prevent 



