38 INTRODUCTION. 



another substance may exert upon one of its components, and what will be the 

 nature and constitution of the product. 



In order to determine the most general laws of Physiological 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 

 extended 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, have 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 king- 

 dom, 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 examina- 

 tion of the phenomena presented by Animals, it is seen that these may be 

 explained better, on the principle that the nervous system has a powerful influ- 

 ence on such actions, than on the idea that it affords a condition essential to 

 them. Recent inquiries have shown, that the agents immediately concerned in 

 these operations are of the same nature in both kingdoms ; the separation of the 

 nutrient materials from the circulating fluid, or the elimination of substances 

 which are to be withdrawn from it, being performed in the Animal, as in the 

 Plant, by cells, in the manner to be explained hereafter. This is only one out 

 of many instances, which it would be easy to adduce, in proof of the necessity 

 of bringing together all the phenomena of the same kind, in whatever class of 

 living beings they may be presented, before we attempt to erect any general 

 principles in Physiology. 



The object of the present treatise, however, is not to follow out such an in- 

 vestigation, which has been pursued, as fully as his limits would allow, in the 

 Author's " Principles of Physiology, General and Comparative;" but to show 

 the detailed application of the principles of which Physiological science may 

 now be said to consist, to the phenomena exhibited by the Human organism, 

 during the continuance of health or normal life. These phenomena, when they 

 occur in a disturbed or irregular manner, constitute disease or abnormal life ; 

 and become the subjects of the science of Pathology. It is impossible to draw 

 a precise line of demarcation between the states of health and disease ; since 

 many variations occur, which do not pass the limits of what must be called in 

 some individuals the normal state, but which must be regarded as decidedly 

 abnormal conditions in others. The sciences ef Physiology and Pathology, 

 therefore, are very closely related; and neither can be pursued with the highest 



