14 INTRODUCTION 



as rational beings, to know the laws by which health and 

 strength may be maintained and disease warded off. 



There are various means by which we gain important infor- 

 mation respecting the Physiology of man. Plants aid us in 

 understanding the minute structure of the human body, its 

 circulation, and absorption. From inferior animals we learn 

 much in respect to the workings of the different organs, as 

 we call those parts of the system which have a particular 

 duty to perform. In one of them, as in the foot of the frog, 

 we can study the circulation of the blood; in another, we 

 can study the action of the brain. 



By vivisection, or the laying bare of some organ of a living 

 animal, we are able to investigate certain vital processes 

 which are too deeply hidden in the human body to be studied 

 directly. This is not necessarily a cruel procedure, as we 

 can, by the use of anaesthetics, so blunt the sensibility of 

 the animal under operation, that he need not suffer while the 

 experiment is being performed. There are other means by 

 which we gather our information. There are occasionally 

 men, who, from some accident, present certain parts, natu- 

 rally out of view, in exposed positions. In these cases, our 

 knowledge is of much greater value than when obtained from 

 creatures lower in the scale of being than man. 



We are greatly aided, also, by the use' of various instru- 

 ments of modern invention. Chief among these is the micro- 

 scope, which is, as we shall learn hereafter, an arrangement 

 and combination of lenses in such a way as greatly to magnify 

 the objects we wish to examine. 



We have much to say of Life, or vital activity, in the course 

 of our study of Physiology ; but the most that we know of it 

 is seen in its results. What Life is, or where its precise posi- 

 tion is, we are not able to determine. We di^rover one thing, 



