INTRODUCTION. 27 



quantities. This seems to be still the most imperfect department of 

 the subject, and one in which we are most liable to hasty conclusions 

 from insufficient data ; but the method is one of great promise, and 

 has already produced much certain and useful information. The animal 

 functions are examined in every way in which they are accessible to 

 physical and numerical investigation. The structure of each organ, 

 and the constituent materials of its tissues, are determined by appro- 

 priate means. The changes in its volume, temperature, vascularity, 

 and composition, the nature and quantity of the materials consumed 

 and of the force manifested, are ascertained and registered. The new 

 substances produced are tested and measured, and the accompanying 

 changes in other organs, or in the whole body, are subjected to similar 

 examination. 



In this way the physiologist studies the living body as a machine. 

 He endeavors to learn the construction of its parts, the mechanism of 

 their action, the materials with which it is supplied, the chemical trans- 

 formations of its internal nutrition, and the phenomena which it ex- 

 hibits in every department of the vital operations. For this purpose 

 he employs all the available means of scientific investigation. 



A large part of the phenomena presented by living creatures are 

 general in character, and show themselves in all classes of vegetable 

 and animal organisms. The absorption of new material and the dis- 

 charge of waste products, indicating the incessant renovation of the 

 organized fabric, and the direct relation between the quantity of nutri- 

 ment consumed and the active manifestation of vitality, are noticeable 

 facts in every form of animated existence. Some of the materials and 

 conditions necessary to life are the same in all cases. The consumption 

 of oxygen and the discharge of carbonic acid are universal phenomena, 

 both in animals and vegetables. The presence and absorption of moist- 

 ure are also indispensable conditions ; and in every case there are 

 certain limits of temperature which cannot be overpassed in either 

 direction without disturbance or arrest of the vital operations. The 

 general nature of these conditions shows their fundamental impor- 

 tance in the phenomena of life, and requires a certain acquaintance 

 with vegetable physiology as an aid to the more successful study of 

 the animal functions. 



On the other hand, there are some' special forms of vital activity 

 which are confined to vegetables, and others which are met with only 

 in animals. Thus, the deoxidation of carbonic acid and water, together 

 with the combination of their remaining elements to form organic mate- 

 rials, can be accomplished only by the living tissues of green vege- 

 tables ; animals having no power to produce organic matter, but only to 

 consume it. Furthermore, it is only in the higher animals that con- 

 sciousness, sensation, and volition appear to have a distinct existence, 

 and come into prominence in connection with the functions of the 

 nervous system. In the animal kingdom certain materials or modes of 

 activity are so nearly the same in many different classes as to indicate 



