14 INTRODUCTION. 



of an infinite wisdom which never errs. We cannot hope to 

 arrive at a knowledge of them by pure reasoning; or by 

 assuming that they are in accordance with definite principles, 

 too often the offspring of our own limited intellects. Never- 

 theless, it is a physiological attribute of the human mind to 

 desire to press on in advance of observation, and to form 

 theories, which may or may not be carried out by the suc- 

 ceeding development of actual knowledge. Theories which 

 are not built upon false or imperfectly observed phenomena, 

 are the pioneers of actual discovery. When theoretical pre- 

 conceptions are justified and corrected by original observa- 

 tions and experiments, with the brain to conceive and the 

 will to execute, man, in thus working out the great problems 

 of Nature, is fulfilling one of the highest purposes of his 

 existence. 



With the few facts which were at first known, the 

 ancient speculative philosophy professed to embrace the 

 whole of natural science ; but as discoveries were made in 

 different departments, a division of labor became necessary. 

 We now find different classes of scientific men, each working 

 in a particular sphere ; as in the lower zoological divisions, 

 a single organ performs all the varied functions of nutrition, 

 while in the higher orders, when the processes of life are 

 more intricate and complicated, the system is divided up 

 into elaborately-organized parts, each of which has an allotted 

 office. 



From the time of Galen may be said to date, as distinct 

 from astronomy, chemistry (or rather alchemy), physics, &c., 

 the science which is now called PHYSIOLOGY. 



Physiology, from its etymology, signifies the science of 

 Nature ; but in the sense in which the term is now used, it 

 may be defined to be the science of life. More elaborate defi- 

 nitions have been given, but they only qualify and explain the 

 meaning of what we know as life. 



A natural division of physiology is into animal and 

 vegetable; and again, into the physiology of the inferior 



