13 INTRODUCTION. 



understand the construction and powers of our bodies ; but 

 it is our duty, 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 

 information respecting the Physiology of man. Plants 

 aid us in understanding the minute structure of the hu- 

 man 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 informa- 

 tion. There are occasionally men, who, from some acci- 

 dent, present certain parts, naturally 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 

 microscope, which is, as we shall learn hereafter, an ar- 

 rangement and combination of lenses in such a way as 

 greatly to magnify the objects we wish to examine. 



