2 INTRODUCTORY [CH. 1. 



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

 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 pro- 

 cesses it will be occasionally necessary to go still further afield, and 

 call the science of vegetable physiology to our assistance. 



The study of physiology must go hand in hand with the study of 

 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 pro- 

 duced 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 shown that 

 the supposition of any special or vital force is unnecessary ; and it 

 should be distinctly recognised that when, in future pages, it is 

 necessary to allude to vital action, it is not because we believe in any 

 specific vital energy, but merely because the phrase is a convenient 

 one for expressing something that we do not fully understand, some- 

 thing that cannot at present be brought into line with the physical 

 and chemical forces that operate in the inorganic world. 



But just as there is no hard-and-fast line between physiology 

 and its allies pathology, anatomy, physics, and chemistry, so also 

 there is no absolute separation between its three great divisions; 

 physical, chemical, and so-called vital processes have to be considered 

 together. 



Physiology is a comparatively young science. Though Harvey 

 more than three hundred years ago laid the foundation of our science 

 by his discovery of the circulation of the blood, it is only during the 

 last sixty or seventy years that active growth has occurred. The 



