4 ELEMENTARY PHYSIOLOGY [less. 



out of the body is hotter and damper tlian the air which 

 is taken in by breathing. 



And lastly ; when we try to ascertain what happens in 

 the eye when that organ is adjusted to different distances ; 

 or what in a nerve when it is excited : or of what materials 

 flesh and blood are made : or in virtue of what mechanism 

 it is that a sudden pain makes one start — we have to call 

 into operation all the methods of inductive and deductive 

 logic ; all the resources of physics and chemistry ; and all 

 the delicacies of the art of experiment. 



The sum of the facts and generalizations at which we 

 arrive by these various modes of inquiry, be they simple 

 or be they refined, concerning the actions of the body and 

 the manner in which those actions ai-e brought about, con- 

 stitutes the science of Human Physiology. An elementary 

 outline of this science, and of so much anatomy as is inci- 

 dentally necessary, is the subject of the following Lessons ; 

 of which we shall devote the present to an account of so 

 much of the structure and such of the actions (or, as they 

 are technically called, " functions ") of the body, as can be 

 ascertained by easy observation ; or might be so ascer- 

 tained if the bodies of men were as easily procured, exa- 

 mined, and subjected to experiment, as those of animals. 



Suppose a chamber with walls of ice, through which a 

 current of pure ice-cold air passes ; the walls of the cham- 

 ber will of course remain unmelted. 



Now, having weighed a healthy living man with great 

 care, let him walk up and down the chamber for an hour. 

 In doing this he will obviously do a considerable amount 

 of work and use up a proportionate quantity of energy ; 

 as much, at least, as would be requii-ed to lift his weight 

 as high and as often as he has raised himself at every step. 

 But, in addition, a certain (juantity of the ice will be 

 melted, or converted into water ; showing that the man 

 has given off heat in abundance. Furthermore, if the air 

 which enters the chamber be made to pass through lime- 

 water, it will cause no cloudy white precipitate of car- 



