208 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, 

 V 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 



