INTRODUCTION. 31 



in their nature from any other chemical phenomena; but they are often 

 different in their conditions and in their results, and are consequently 

 peculiar and characteristic. 



Another set of vital phenomena are those whiph are manifested in the 

 processes of reproduction and development. They are entirely distinct 

 from any phenomena which are exhibited by matter not endowed with 

 life. An inorganic substance, even when it has a definite form, as, for 

 example, a crystal of fluor spar, has no particular relation to any similar 

 form which has preceded, or any other which is to follow it. On the 

 other hand, every animal and every vegetable owes its origin to pre- 

 ceding animals or vegetables of the same kind; and the manner in 

 which this production takes place, and the different forms through 

 which the new body successively passes in the course of its develop- 

 ment, constitute the phenomena of reproduction. These phenomena 

 are mostly dependent on the chemical processes of nutrition and growth, 

 which take place in a particular direction and in a particular manner ; 

 but their results, namely, the production of a connected series of different 

 forms, constitute a separate class of phenomena, which cannot be ex- 

 plained in any manner by the preceding, and require, therefore, to be 

 studied by themselves. 



Another set of vital phenomena are those which belong to the nervous 

 system. These, like the processes of reproduction and development, 

 depend on the chemical changes of nutrition and growth. That is to 

 say, if the nutritive processes did not go on in a healthy manner and 

 maintain the nervous system in a healthy condition, the peculiar phe- 

 nomena which are characteristic of it could not take place. The nutri- 

 tive processes are necessary conditions of the nervous phenomena. But 

 there is no other connection between them ; and the nervous phenomena 

 themselves are distinct from all others, both in their nature and in the 

 mode in which they are to be studied. 



The study of Physiology is naturally divided into three distinct Sec- 

 tions : 



I. The first of these includes everything which relates to the NUTRI- 

 TION of the body in its widest sense. It comprises the history of the 

 proximate principles, their source, the manner of their production, the 

 proportions in which they exist in different kinds of food and drink, the 

 processes of digestion and absorption, and the constitution of the circu- 

 lating fluids ; then, the physical phenomena of the circulation and the 

 forces by which it is accomplished ; the changes which the blood under- 

 goes in different parts of the body ; all the phenomena, both physical 

 and chemical, of respiration; those of secretion and excretion, and the 

 character and destination of the secreted and excreted fluids. All these 

 processes have reference to a common object, namely, the preservation of 

 the normal structure and organization of the individual. Their results 

 cprnprise the phenomena of internal growth and nutrition, which are 

 common to the animal and vegetable kingdoms; and they are accord- 

 ingly known by the name of the vegetative functions. 



