INTRODUCTION. 55 



phenomena, though strictly physical in character, are often entirely 

 peculiar and different from those seen elsewhere, because the me- 

 chanism 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 

 flexible mucous membrane, and the contractile muscles outside, are 

 of such a special character that the entire apparatus, as well as the 

 sounds produced 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 compli- 

 cated and remarkable that they cannot be comprehended, even by 

 one who is acquainted with the anatomy of the organ, without a 

 direct examination. This is not because there is anything essen- 

 tially obscure or mysterious in their nature,, for they are purely 

 mechanical in character ; but because their conditions are so pecu- 

 liar, owing to the tortuous course of the muscular fibres, their 

 arrangement in interlacing layers, their attachments and relations, 

 that their combined action produces an effect altogether 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 conditions of the existence of every animal 

 and every vegetable, that it should constantly absorb various sub- 

 stances from without, which undergo different chemical alterations 

 in its interior, and are finally discharged from it under other forms. 

 If these changes be prevented from taking place, life is immediately 

 extinguished. Thus animals constantly absorb, on the one hand, 

 water, oxygen, salts, albumen, oil, sugar, &c., and give up, on the 

 other hand, to the surrounding media, carbonic acid, water, ammonia, 

 urea, and the like ; while between these two extreme points, of ab- 

 sorption and exhalation, there take place a multitude of different 

 transformations which are essential to the continuance of life. 



Some of these chemical actions are the same with those which 

 are seen outside the body ; but most of them are entirely peculiar, 

 and do not take place, and cannot be made to take place, anywhere 

 else. This, again, is not because there is anything particularly 

 mysterious or extraordinary in their nature, but because the con- 



