50 INTRODUCTION. 



is naturally divided into two parts, viz., Vegetable Physiology, and 

 Animal Physiology. 



Again, the different groups and species of animals, while they 

 resemble each other in their general characters, are distinguished 

 by certain minor differences, both of structure and function, which 

 require a special study. Thus, the physiology of fishes is not 

 exactly the same with that of reptiles, nor the physiology of birds 

 with that of quadrupeds. Among the warm-blooded quadrupeds, 

 the carnivora absorb more oxygen, in proportion to the carbonic 

 acid exhaled, than the herbivora. Among the herbivorous quad- 

 rupeds, the process of digestion is comparatively simple in the 

 horse, while it is complicated in the ox ; and other ruminating ani- 

 mals. There is, therefore, a special physiology for every distinct 

 species of animal. 



HUMAN PHYSIOLOGY treats of the vital phenomena of the human 

 species. It is more practically important than the physiology of 

 the lower animals, owing to its connection with human pathology 

 and therapeutics. But it cannot be made the exclusive subject of 

 our study ; for the special physiology of the human body cannot 

 be properly understood without a previous acquaintance with the 

 vital phenomena common to all animals, and to all vegetables; 

 beside which, there are many physiological questions that require 

 for their solution experiments and observations, which can only be 

 made upon the lower animals. 



While the following treatise, therefore, has for its principal sub- 

 ject the study of Human Physiology, this will be illustrated, when- 

 ever it may be required, by what we know in regard to the vital 

 phenomena of vegetables and of the lower animals. 



II. Since Physiology is the study of the active phenomena of 

 living bodies, it requires a previous acquaintance with their struc- 

 ture, and with the substances of which they are composed ; that is, 

 with their anatomy. 



Anatomy, again, requires a previous acquaintance with inorganic 

 substances ; since some of these inorganic substances enter into the 

 composition of the body. Chloride of sodium, for example, water, 

 and phosphate of lime, are component parts of the animal frame, 

 and therefore require to be studied as such by the anatomist. 

 Now these inorganic substances, when placed under the requisite 

 external conditions, present certain active phenomena, which are 

 characteristic of them, and by which they may be recognized. 



