218 On the Structure and Functions of the Organs of Respiration. 



If we submit to analysis the series of changes which constitute the phae- 

 nomena of life, we shall find that they may be arranged under certain 

 classes, to which we give the name of Junctions : some of these, as diges- 

 tion, circulation, respiration, &c. are essential to the preservation of all 

 organic systems, whether in the animal or vegetable kingdoms, and are 

 hence called functions of organic life ; whilst others, as sensation and loco- 

 motion, are confined to animals, and are termed functions of animal life ; 

 and a third class, which exists in all organised bodies, has for its object the 

 continuance of the species. In the lowest orders of being, we find these 

 functions very much blended together, and several of them apparently 

 performed by one simple apparatus j but in proportion as we rise in the 

 scale, we perceive that they are specialized, or separated from each other, 

 and that a complicated set of organs is appropriated to each of them. 

 They all appear to be more or less under the dominion 'of the common 

 laws of attraction and affinity ; but whether the extraordinary effects, 

 which we may admire and investigate, but which we cannot imitate, are 

 owing to a modification of these laws by vital power, or only to the peculiar 

 manner in which they are brought into action, is a question with regard 

 to which physiologists have as yet scarcely a right to give a decided an- 

 swer. Probably, however, the latter is the correct mode of viewing the 

 subject ; and as the function of respiration is one, which, from its compa- 

 rative simplicity, is pretty well understood, we have selected it as an illus- 

 tration of these principles : it may be hoped that the time is not far 

 distant, when our increased acquaintance with organic chemistry may 

 enable us to submit other functions, such as those of digestion and secre- 

 tion, to a similar analysis. 



All living bodies, both animals and vegetables, are composed of solid and 

 liquid constituents, the former taking their origin from the latter, and 

 being constantly maintained by them ; they are in intimate connexion 

 with each other, and the continuance of life depends on their reciprocal 

 action. In the higher forms of organic life, we discover a special appa- 

 ratus for transmitting throughout the body the nutritive fluid, elaborated 

 by the digestive organs ; but as we descend in the scale, we find the vas- 

 cular system in a less isolated form, and in some instances we lose all 

 traces of distinct vessels for the circulation of fluid. 



Respiration may be defined — the function by which the fluid, which has 

 thus ramified through die body, is exposed to the action of atmospheric 

 air, (either in a pure state, or as contained in water,) by which action a 

 change in its chemical constituents takes place, its vital properties are 

 renovated, and it is refitted for its essential duty, the nutrition of the sys- 

 tem. It is not, however, to be supposed, that this fluid is ever brought 

 into immediate contact with the air ; a thin membrane intervenes in all 

 instances ; and the manner in which the process of aeration is effected, 

 had long been a difficult question among both animal and vegetable physio- 



