ANIMAL. 



143 



immediate nourishment of every part of the 

 organization. 



The different media in which animals live 

 involves the supposition of another modifica- 

 tion as to the mode in which the blood or 

 nutritive fluid is aerated. Those that live in 

 air respire this elastic fluid immediately; those 

 that live in water, again, respire it mingled 

 with or dissolved in the surrounding medium. 

 The tracheae of those animals whose respira- 

 tion is diffuse, and that exist on the surface 

 of the earth, consequently are filled with air; 

 those of the creatures that exist in water are 

 conduits for the constant transmission of this 

 fluid. When the respiration is concentrated, 

 corresponding modifications in the function 

 are encountered according to the medium in 

 which animals live : the air is either received 

 immediately into the body, when the apparatus 

 is known as a lung, or, suspended among 

 water, it is passed over the surface of the 

 respiratory organ, which is then denominated 

 gilL Quadrupeds and birds respire univer- 

 sally by means of lungs, fishes and the mol- 

 lusca by means of gills. In certain reptiles 

 the function is carried on by means both of 

 lungs and gills, and as it would appear even 

 by the general surface of the body either vica- 

 riously, or at one and the same time. These 

 are the only true amphibious animals. 



A circulation, properly so called, is the ap- 

 panage of an organization already somewhat 

 complicated, consequently of an animal con- 

 siderably raised in the scale of creation. This 

 function, it is evident, as implying in its sim- 

 plest sense a progressive motion of the general 

 nutritive fluid or blood, can only exist where 

 such a fluid is encountered. It is altogether 

 wanting, therefore, among those animals in 

 which nutrition is accomplished immediately. 

 We ascend but a little way in the scale before 

 we find the function consisting not only of an 

 outward or progressive motion of the nutritive 

 fluids, but of a retrograde motion also of these 

 same fluids modified in their nature, and re- 

 quiring exposure to a greater or less degree in 

 some form of respiratory apparatus to fit them 

 anew for distribution to the organization at 

 large. The fluid in this instance parts from 

 a centre, and returns thither after having made 

 the round of the system. Circulation in this 

 acceptation only occurs among those animals 

 that have a separate respiratory apparatus, and 

 in which we meet with absorption of nutri- 

 ment from without, and of lymph, &c. from 

 within. The pabulum of nutrition is taken 

 up by lacteals and veins from the digestive 

 apparatus, and by veins and lymphatics from 

 the rest of the organism for transmission, under 

 the name of venous blood, to the apparatus of 

 respiration, whatever its form. In this the 

 fluid, still immature and unapt for assimilation, 

 is exposed in vessels of infinite minuteness 

 and extreme tenuity to the action of the at- 

 mospheric air, and having undergone in these 

 a certain change, it begins to be collected by 

 another set of vessels, which form branches suc- 

 cessively of larger and larger size, until finally 

 it is projected from the respiratory apparatus in 



one or more trunks, under the name of arterial 

 blood, fitted for assimilation by the organization 

 at large, and proving the principal stimulus 

 under the influence of which its various par- 

 ticular organs accomplish their offices. 



Circulation, however, as a function, is com- 

 plicated in the same degree as the apparatus 

 by which it is effected. In some classes we 

 find the circulation taking placing through 

 vessels only, one set distributing the blood from 

 the respiratory apparatus to the body generally, 

 another collecting this fluid again, and the 

 newly-absorbed matters from the body at large, 

 and transmitting these for elaboration anew in 

 the organ of respiration. In other tribes, and 

 this invariablj' after the very lowest grades of 

 the scale are passed, we find the hollow muscle, 

 or forcing apparatus, which, in glancing at the 

 differences of structure, we have spoken of 

 as the heart superadded to the circle of vessels, 

 which even in its simplest state consists of 

 at least two cavities communicating with one 

 another, one for the reception of the blood 

 from, the other for the projection of this fluid 

 to the general system. 



But the blood does not follow the direct 

 and simple course here supposed in almost 

 any case. There is the aeration of the fluid 

 in the way, and means to accomplish this 

 important end must of course be provided. 

 Among many animals it would appear by 

 no means necessary that the whole of the 

 blood should undergo exposure in the respira- 

 tory apparatus, in order to fit it for the wants 

 of the organization ; a part only'is sent thither, 

 and this on admixture with the remainder 

 suffices to revivify the mass. In this case it 

 is not imperative that the two kinds of blood 

 the unaerated or venous, and the aerated or 

 arterial should be kept distinct; there is con- 

 sequently no occasion for more than one re- 

 cipient cavity or auricle, into which the aerated 

 blood from the organ of respiration, and the 

 unaerated blood of the system are poured in 

 common and mingled, and one projecting 

 cavity or ventricle from which the mixed cur- 

 rent is distributed partly to the respiratory ap- 

 paratus and partly to the system at large. 

 Here the blood in its course describes no more 

 than a single circle, beginning and ending in 

 the heart, which is then characterized as simple, 

 consisting, as has been said, of a single auricle 

 and a single ventricle. Among other tribes of 

 animals, however, the whole mass of blood 

 requires to undergo aeration in the respiratory 

 apparatus each time it completes its round 

 before it can again subserve the wants of the 

 organization. In this instance it is evident that 

 the aerated and unaerated blood require to be 

 most particularly prevented from commingling, 

 and that a single or simple heart will no longer 

 suffice as the implement of circulation. This 

 complex circulation is met with among ani- 

 mals so low in the scale as to be unprovided 

 with a heart, when of course it is accomplished 

 by means of vessels only. In some tribes the 

 one portion of the function is performed by the 

 medium of vessels, the other by the agency of 

 a heart which is now connected with the gene- 



