INTRODUCTION. 



THE object of the science of Physiology is to bring together, in a sys- 

 tematic form, the phenomena which normally present themselves during the 

 existence of living beings; and to classify and compare these, in such a man- 

 ner as to deduce from them the general laws or principles, according to which 

 they take place. 



The term Law having been frequently applied to physical and physiological 

 phenomena, in a manner very different from that which sound philosophy 

 sanctions, it is desirable to explain the acceptation (believed by the author to 

 be the only legitimate one) in which it is here employed. The so-called 

 Laws of Nature are nothing else than general expressions of the conditions, 

 under which certain assemblages of phenomena occur ; so far as those con- 

 ditions are known to us. Thus the law of Gravitation, in General Physics 

 (the most universal in its action of any with which we are acquainted), is 

 nothing else than a simple expression of the fact, that, under all circum- 

 stances, two masses of matter will attract each other with forces directly pro- 

 portional to their respective bulks, and inversely as their distances. So, again, 

 the law of Cell-growth, which seems to hold the same rank in Physiology 

 with that of Gravitation in Physics, embodies these two general facts, that 

 all organised beings originate in cells, and that the various functions of life 

 are carried on, even in the adult condition, by the continued growth and de- 

 velopment of cells. 



In no case can natural phenomena be correctly said to be governed by 

 laws ; since the laws themselves are nothing else than manifestations of the 

 Will of the governing Power. But they may be properly said to take place 

 according to certain laws ; these laws being framed by Man as expressions 

 or descriptions of the slight glimpses he possesses, of the plan according to 

 which the Creator sees fit to operate in the natural world. Thus understood, 

 the use of the term Law can be in no way supposed to imply, that the Deity 

 stands in any other relation to the phenomena of the Universe than as their 

 direct and constantly-operating Cause. 



In order to determine the true laws, or most general principles, of Phy- 

 siological Science, a very extensive comparison is requisite. Principles, which 

 might seem of paramount importance in regard to one group of living beings, 

 are often found, on a more general review, to be quite subordinate. For 

 example, the predominance of the Nervous System in the higher classes of 

 Animals, and its evidently close connection with many of the functions of 

 life, has led several Physiologists to the opinion, that its influence is essential 

 to the performance of the functions of Nutrition, Secretion, &c. ; but, on 

 turning our attention to the Vegetable kingdom, in which nothing analogous 

 to a nervous system can be proved to exist, we find these functions going on 

 with even greater activity than in animals. It is clear, therefore, they may 

 be performed without it ; and, on a closer examination of the phenomena 

 4 



