ELEMENTARY INSTRUCTION IN PHYSIOLOGY. 97 



is eminently characteristic of physiology the difficulty 

 of the subject begins beyond the stage of elementary 

 knowledge, and increases with every stage of progress. 

 While the most highly trained and the best furnished 

 intellect may find all its resources insufficient, when it 

 strives to reach the heights and penetrate into the depths 

 of the problems of physiology, the elementary and fun- 

 damental truths can be made clear to a child. 



No one can have any difficulty in comprehending 

 the mechanism of circulation or respiration ; or the gen- 

 eral mode of operation of the organ of vision ; though 

 the unravelling of all the minutiae of these processes, 

 may, for the present, baffle the conjoined attacks of the 

 most accomplished physicists, chemists, and mathema- 

 ticians. To know the anatomy of the human body, 

 with even an approximation to thoroughness, is the work 

 of a life; but as much as is needed for a sound com- 

 prehension of elementary physiological truths, may be 

 learned in a week. 



A knowledge of the elements of physiology is not 

 only easy of acquirement, but it may be made a real 

 and practical acquaintance with the facts, as far as it 

 goes. The subject of study is always at hand, in one- 

 self. The principal constituents of the skeleton, and 

 the changes of form of contracting muscles, may be felt 

 through one's own skin. The beating of one's heart, 

 and its connection with the pulse, may be noted; the 

 influence of the valves of one's own veins may be 

 shown ; the movements of respiration may be observed ; 



