On the Structure and Functions of the Organs of Resp'iraUon. 221 



of respiration does not depend on the comparative size of the organs, but 

 on the extent of surface by which the blood is exposed to the action of the 

 air; so that the minutely subdivided cellular lungs of a rabbit present 

 greater opportunity for the aeration of the blood than the enormous sacs of 

 a turtle leu times its size. 



The organs appropriated to the performance of the function of respira- 

 tion appear, at first sight, so very different, that a superficial observer would 

 liardly trace any analogy between them. There would seem, for instance, 

 but little resemblance between the gills of a fish, and the lungs of a quad- 

 ruped ; or between the beautiful tufts on the body of a sand-worm, and the 

 air tubes of an insect : but a little consideration will show that all these 

 forms are reducible, as in vegetables, to one simple element, — an exten- 

 sion of the external surface, specially modified l)y its permeability and 

 its vascularity for the aeration of the blood. This extension usually takes 

 place externally or internally, according as the animal inhabits a fluid or 

 aeriform medium ; the circulating fluid, in aquatic respiration, being ge- 

 nerally propelled outwards to meet the air contained in the water ; and 

 in atmospheric respiration, the air entering the system to meet the fluid : 

 the comparative densities of these media afford obvious reasons for this 

 modification. 



In the lowest classes of animals, which are all aquatic, we find the 

 aeration of the fluid parts of the body performed by the external surface, 

 and in some instances, apparently, by that internal prolongation of it 

 which forms the digestive canal. The vibratile cilia, with which we see 

 most infusorial animalcules, as well as the higher kinds of zoophytes, so 

 largely provided, seem essentially connected with the function of respira- 

 tion ; and they may be traced upwards in the respiratory apparatus of the 

 higher invertebrata, and even in some of the vertebrated animals. They do 

 not however appear designed to affbrd an increase of surface for the distri- 

 bution of capillary vessels ; but, by their constant vibration, to renew the 

 stratum of water in contact with the body, and thus to bring air as well as 

 food within the animal's reach. "The simple organisation of these ani- 

 mals," says Dr. Grant, " requires no other respiratory system. Their 

 " bodies are not yet covered with solid shells, or with dense impervious 

 " scales, or with other hard materials which would exclude the general rc- 

 "spiratory influence of water, and necessitate the formation ofgills or lungs; 

 " but consist of the soft cellular tissue in which all higher organisations are 

 " first developed. The few kinds which are furnished with a tiiin transparent 

 " horny pellicle, have the power of extending the ciliated and prehensile 

 "part of their body from beneath it; and thus of effecting all the required 

 " respiration." 



In sponges and other poriferous animals, which have a similar soft cellu- 

 lar covering, observation of their living state shows the rapidity with 

 wliich the water is renewed over the surface of the body, and throughout 



No. 4.— Vol. I. 2 o 



