INTRODUCTION. 29 



quantity and distribution in the animal frame, their occurrence as con- 

 stituent parts of the food, their combinations and decompositions in the 

 body, and the form under which they appear in the products of excre- 

 tion. It aims to give a general view of the materials supplied to the 

 animal organism, and the use which they subserve in the processes 

 of life. 



The second section treats of the functions of Nutrition. It includes 

 the action of the digestive apparatus, by which the food is prepared 

 for assimilation, the absorption of the digested products, their elabora- 

 tion in the glandular organs, the blood and its circulation, the forma- 

 tion and character of the secretions, the phenomena of respiration, the 

 production of animal heat, and the constitution and properties of the 

 excreted fluids. These processes have for their object the vegetative 

 growth and renovation of the body, or the maintenance of its normal 

 structure and organization. They are for the most part of a physical 

 or chemical nature, and are distinguished from other physical or chem- 

 ical phenomena only by the variety and complexity of their results. 



The third division in the natural order of study embraces the func- 

 tions of the Nervous System. These functions are of a different 

 character from the preceding, and are investigated by different means. 

 The two groups of phenomena are thus distinguished from each other, 

 notwithstanding the fact that they are mutually dependent. The 

 activity of the nervous system requires for its support a continued 

 nutrition; and on the other hand the influence of the nervous system is 

 everywhere felt by the organs of circulation and secretion. But the 

 immediate action of the nervous system is, so far as we can judge, of 

 a special nature, and one which has no resemblance to the nutritive 

 operations. It is a means of sympathetic communication, by which the 

 different organs are alternately stimulated or controlled, and which acts 

 as the instrument of sensibility, consciousness, volition, and movement. 

 It brings the living body into active relation with the external world, 

 and provides for the exercise of the animal instincts and powers. 



The last group of functions contains those belonging to Reproduc- 

 tion. They are made up of phenomena, different in kind from either 

 of the foregoing, and having for their object the continuation of the 

 species. They consist in the production, from the parent organism, 

 of the sexual elements, and in the appearance, from the union of these 

 elements, of a progressive series of organic forms, following each 

 other in a determinate order of successive transformations, until the 

 last form in the series reproduces that of the original parent. The 

 distinguishing feature of this process is therefore that the functions 

 of nutrition and growth are here directed by a law of continuous devel- 

 opment ; and it presents, as the main object of our study, the form and 

 structure of the different parts as they successively appear in the grow- 





