298 INSTRUCTION IN PHYSIOLOGY xi 



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's self. The principal constituents 

 of the skeleton, and the changes of form of con- 

 tracting muscles, may be felt through one's own 

 skin. The beating of one's heart, and its connec- 

 tion 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 ; 

 while the wonderful phenomena of sensation 

 afford an endless field for curious and interesting 

 self-study. The prick of a needle will yield, in a 

 drop of one's own blood, material for microscopic 

 observation of phenomena which lie at the found- 

 ation of all biological conceptions; and a cold, 

 with its concomitant coughing and sneezing, may 

 prove the sweet uses of adversity by helping one 

 to a clear conception of what is meant by " reflex 

 action." 



Of course there is a limit to this physiological 

 self-examination. But there is so close a solidar- 

 ity between ourselves and our poor relations of 

 the animal world, that our inaccessible inward 

 parts may be supplemented by theirs. A com- 

 parative anatomist knows that a sheep's heart and 



