14 HORSE-BACK RIDING. 



movements, or movements in situ, and which we ob- 

 serve in man with a degree of frequency and com- 

 plexity, varied almost ad infinitimt, consist in the 

 change of relation of the divers segments which com- 

 pose the skeleton : changes of situation, by virtue of 

 which the members play the most important part, 

 although the trunk itself generally participates in the 

 motion. 



But even when man or the animals do not execute 

 the extended movements of which we have spoken, 

 they are still far from being immobile. The thoracic 

 cage is each instant raised and lowered, moved by the 

 filling of the lungs, and by their return to their first 

 dimensions, the entrance and exit of the air necessary 

 to respiration. The digestive tube and the stomach 

 work upon the aliments contained in their cavity. 

 At certain moments, which correspond with the feel- 

 ing of hunger and thirst, food is brought to the 

 mouth and taken by it ; the tongue, the teeth, 

 jaws . and pharynx set to work each in their way, 

 to divide the food, to masticate and swallow it, 

 etc. . . . And when digestion is accomplished, 

 the residue is expulsed by the active forces of defeca- 

 tion. 



At every movement the heart contracts on the 

 blood which is brought to it, and sends it to the 

 arteries. The arteries, capillaries, and veins work 

 upon that liquid by a retrograde movement due to 

 the elasticity of their walls, and also, in certain con- 



