INTRODUCTION. 29 



expended in its manifestation. The study of physiology, therefore, 

 requires a certain knowledge of the chemical and physical reactions 

 presented in the outer world, in order that the observer may be able to 

 appreciate the peculiarities of similar phenomena as they occur in the 

 living body. As all animated beings are closely dependent on external 

 conditions for the maintenance of their vitality, it is evident that the 

 study of their vital actions cannot be disconnected from that of external 

 natural phenomena. The pressure and tension of the atmosphere, for 

 example, as well as its chemical constitution, are directly connected with 

 the process of respiration ; and the circulation of the blood through the 

 vessels exhibits the physical phenomena of an incompressible fluid flow- 

 ing through elastic tubes. 



By the term vital phenomena, accordingly, we mean those phenomena 

 which are manifested in the living body, and which are characteristic of 

 its functions. At the same time many of them do not differ in character 

 from those of the outside world, but only in the peculiarity of their 

 conditions and their results. 



Some of these phenomena are physical or mechanical in their charac- 

 ter ; as, for example, the play of the articulating surfaces upon each 

 other, the balancing of the spinal column with its appendages, the action 

 of the elastic ligaments. Nevertheless, these phenomena, though strictly 

 physical in character, are often entirely peculiar and different from those 

 seen elsewhere, because the mechanism of their production is peculiar in 

 its details. Thus the human Voice and its modulations are produced in 

 the larynx, in accordance with the general physical laws of sound ; but 

 the arrangement of the elastic and movable vocal chords, and their 

 relations with the columns of air above and below, the moist and flexi- 

 ble mucous membrane, and the contractile muscles outside, are of such 

 a special character that the entire apparatus, as well as the sounds pro- 

 duced by it, is peculiar ; and its action cannot be properly compared 

 with that of any other known musical instrument. 



In the same manner, the movements of the heart are so complicated 

 and remarkable that they cannot be comprehended, even by one who is 

 acquainted with the anatoniy of the organ, without a direct examination. 

 This is not because there is anything essentially obscure or mysterious 

 in their nature, for they are purely mechanical in character ; but because 

 their conditions are so peculiar, owing to the tortuous course of the 

 muscular fibres, their arrangement in interlacing layers, their attach- 

 ments and relations, that their combined action produces an effect alto- 

 gether peculiar, and one which is not similar to anything outside the 

 living body. 



A very large and important class of the vital phenomena are those of 

 a chemical character. It is one of the characteristics of living bodies 

 that a succession of chemical actions, combinations, and decompositions, 

 is constantly going on in their interior. It is one of the necessary con- 

 ditions of the existence of every animal and every vegetable, that it 

 should constantly absorb various substances from without, which under- 



