362 OF RESPIRATION. 



absorbed by the liquid. In this manner, as already mentioned, the 

 surface of venous blood, inclosed in a bladder, may be made to exhibit 

 the arterial hue, by suspending the bladder in an atmosphere of 

 oxygen ; for the carbonic acid of the blood, and the surrounding 

 oxygen, will overcome by their mutual attraction the obstacle inter- 

 posed by the bladder ; and the former will be lifted out, so to speak, and 

 will be replaced by the latter. It has been found by experiment, that 

 the free carbonic acid diffused through blood, may be more completely 

 extricated from the liquid, by exposing it to hydrogen, than by placing 

 it under the vacuum of an air-pump ; for in the latter case there is 

 nothing to replace it, and the attraction between the gas and the liquid 

 tends to resist the exhausting influence of the vacuum; whilst in the 

 former, the blood receives one gas in exchange for the other, so that 

 the whole force of the tendency to mutual diffusion is exercised in lift- 

 ing out the carbonic acid. 



. 651. The immediate purpose of the organs of Respiration, then, 

 whatever may be the variety in their form, is this : to expose the 

 blood to the air, in a state of such minute division as to present a very 

 extended surface, a thin membrane only being interposed between 

 them. For this purpose we find a certain organ, or set of organs, spe- 

 cially set apart in all the higher animals ; and this is formed by a 

 prolongation of the general surface, either externally or internally, 

 according to the mode in which the respiration is accomplished. Thus 

 in Fishes and aquatic Molluscs, the blood is aerated by exposure, not 

 directly to the atmosphere, but to the air which is dissolved in the 

 water they inhabit ; and the respiratory apparatus is formed in them 

 of an extension of the external surface, at a particular part, into innu- 

 merable delicate fringe-like processes, the gills (Fig. 100) ; every divi- 

 sion of which contains a network of blood-vessels (Fig. 104) ; so that 



Fig. 100. 



Doris Johnstoni, a Nudibranchiate Gasteropod, showing the tuft of external gills. 



the amount of blood which is exposed to the surrounding medium at 

 any one time, is collectively very great, although the quantity contained 

 in each gill-filament is very minute. On the other hand, in all the air- 

 breathing Vertebrata, the blood is exposed to the atmosphere, through 

 the medium of an internal membranous prolongation, which is conti- 

 nuous with the mucous membrane lining the mouth and nostrils ; this 

 forms a pair of sacs, termed lungs, communicating with the back of the 

 mouth by means of a tube called the trachea or windpipe, through 

 which air is freely admitted to the cavities thus formed (Fig. 105). 



