'General Principles of Physiology. 97 



I have another motive, however, for undertaking the task I 

 ■have imposed on myself. It appears to me that when the whole 

 of the facts are brought together, we shall be led to inferences 

 of considerable importance in the animal economy, which a 

 more partial tiew cannot afford. 



The animal differs from the vegetable worUl in possessing 

 sensation and volition. These functions we shall find are 

 essential to the continuance of life, at least in the higher 

 classes of animals, and consequently as far as they are so, 

 belong to the department of the physiologist, the other func- 

 tions of the mind which form no part of the powers by which 

 life is preserved, belonging exclusively to that of the moralist. 

 By the term sensorial powers, therefore, which I shall employ 

 for the sake of brevity, I mean sensation and volition alone. 



Both the animal and vegetable world differ from inanimate 

 matter, in affording a peculiar class of results when impressed 

 by other agents, whether chemical or mechanical. The quality 

 on which the peculiarity of these results depends, has been 

 termed the vital principle. Whether this principle be some- 

 thing superadded to bodies, or only a peculiar arrangement of 

 their constituent parts, we have no means of ascertaining. 

 The fact is, that it bestows on matter certain properties. It 

 is essential that its name should convey this fact, and no more. 



The phenomena of the vital principle have been observed 

 and arranged with more care in the animal, than the vegetable 

 world. Of the former alone, notwithstanding the intimate con- 

 nexion of the subjects, I propose to treat here. They may be 

 divided into two classes, those of the muscular, and those of 

 the nervous system. 



I shall begin with the muscular system, because its function 

 is the simplest. It consists merely in a contractile power, by 

 which, in consequence of the impression of certain agents, the 

 extremities of the muscle are made to approach each other, 

 its firmness and transverse diameter being increased in a 

 degree which bears a certain proportion to the approximation 

 of its extremities. 



Previous to the time of Haller, it had been universally sup- 

 VoL. XIII. H 



