INTRODUCTORY. [CH. I. 



point out from time to time the practical relationships between 

 physiology and pathology. 



Human physiology will be our chief theme, but it is not a por- 

 tion of the great science that can be studied independently of its 

 other portions. Thus, many of the experiments upon which our 

 knowledge of human physiology rests have been performed 

 principally on certain of the lower animals. In order to obtain a 

 wide view of vital processes it will be occasionally necessary to go 

 still further afield, and call the science of vegetable physiology to 

 our assistance. 



In another sense, human physiology is in no isolated position. 

 Its study must go hand in hand with the study of anatomy. It 

 is impossible to understand how the body or any part of the body 

 acts unless we know accurately the structure of the organs under 

 consideration. This is especially true for that portion of anatomy 

 which is called Microscopic Anatomy or Histology. Indeed, so 

 close is the relationship between minute structure and function 

 that in this country it is usual for the teacher of physiology to 

 be also the teacher of histology. Another branch of anatomy, 

 namely, Embryology, or the process of growth of the adult from 

 the ovum, falls also within the province of the physiologist. 



But physiology is not only intimately related in this way to 

 its sister science anatomy, but the sciences of chemistry and 

 physics must also be considered. Indeed, physiology has been 

 sometimes defined as the application of the laws of chemistry and 

 physics to life. That is to say, the same laws that regulate the 

 behaviour of the mineral or inorganic world are also to be found 

 operating in the region of organic beings. If we wish for an 

 example of this we may again go to the eye ; the branch of 

 physics called optics teaches us, among other things, the manner 

 in which images of objects are produced by lenses; these same 

 laws regulate the formation of the images of external objects upon 

 the sensitive layer of the back of the eye by the series of lenses 

 in the front of that organ. An example of the application of 

 chemical laws to living processes is seen in digestion ; the food 

 contains certain chemical substances which are acted on in a 

 chemical way by the various digestive juices in order to render 

 them of service to the organism. 



The question arises, however, is there anything else ? Are there 

 any other laws than those of physics and chemistry to be reckoned 

 with ? Is there, for instance, such a thing as " vital force " ? It 

 may be frankly admitted that physiologists at present are not 

 able to explain all vital phenomena by the laws of the physical 

 world ; but as knowledge increases it is more and more abundantly 



