12 . HUMAN PHYSIOLOGY 



precede physiology is to have them simultaneous with it. 

 If only one can be taken the choice must fall on chemistry. 



The teacher who is called upon to give a course in 

 physiology to students who have had neither -physics nor 

 chemistry is severely handicapped. They must soon be 

 using phrases which cannot mean to them what they mean 

 to others of ampler training. They may recite glibly 

 and to the satisfaction of an easy-going instructor, but 

 the large conceptions may be wanting. A teacher who 

 has to make the most of such a situation will be com- 

 pelled to sacrifice much in other directions to secure clear- 

 ness as to fundamentals. Detail, however attractive, 

 must give way to cardinal truths. 



What are the matters which must be impressed at any 

 cost? First of all, the conservation of energy and its 

 convertibility from one form to another. Second, the 

 closely related fact of the latency of energy in those com- 

 pounds which we call fuels. The recognition of food as a 

 biologic fuel. The general significance of oxidation and 

 the release of potential energy. The realization that the 

 respiratory process is a particular case of oxidation and 

 that its value is in the setting free of energy that becomes 

 manifest as heat and mechanical work. Finally, the 

 conception that the development and application of 

 energy are determined by stimuli brought to bear upon 

 organisms from the world without. 



Attention may be called to one of the many sources of 

 confusion which troubles beginners. This is the ques- 

 tion of scale. The student has thought almost altogether 

 in the past of things which are appreciated by the naked 

 eye. When he enters upon his study of physiology he 

 may be said to be asked to conceive of three orders of 

 magnitude. The features of gross anatomy present 

 no difficulty. But these are pictured on one page of his 

 book while on the next there may be a representation of 

 cells. The teacher must be at pains to make plain what 

 ratio obtains between the two. 



The following device may be helpful. A fine hair may 



