RESPIRATION IN MOLLUSCS. 863 



The blood is minutely distributed on the walls of these sacs by a close 

 network of Capillary vessels (Fig. 106) ; and not only on the external 

 walls, but also on numerous partitions by which the cavities are sub- 

 divided with more or less minuteness, so as greatly to extend the vas- 

 cular surface. 



652. Such is the essential nature of the Respiratory apparatus ; but 

 in order that it may be carried into that vigorous operation, which is 

 required in the higher classes of animals, various supplementary arrange- 

 ments are made, for the purpose of promoting the due influence of the 

 air upon the blood. In the first place, the capillary vessels of the respi- 

 ratory surface are connected with arterial trunks, which issue immedi- 

 ately from the heart, and which thus convey a constant stream of blood 

 from that organ ; whilst they give origin to venous trunks, which 

 terminate directly in the heart, and which are ready to convey back to 

 it the blood that has undergone aeration. Thus by the energetic action 

 of the heart, and by the force generated in the capillaries of the lungs 

 ( 598), a constant renewal is effected in the blood, which is exposed to 

 the air through the medium of these organs. On the other hand, the 

 renewal of the blood would be useless, unless a, fresh supply of air were 

 continually being introduced, and that which had been vitiated, by the 

 loss of its oxygen and the admixture of carbonic acid, were removed ; 

 and this is effected by a series of muscular movements, which are adapted 

 for the alternate expulsion of the vitiated air from the lungs, ,and for 

 the introduction of a fresh supply of pure air from the atmosphere. 

 These movements are kept up by a certain part of the nervous system ; 

 but they are not dependent upon any exertion of the will, for they con- 

 tinue during profound sleep, and in other states in which even conscious- 

 ness is altogether suspended. 



2. Different forms of the Respiratory Apparatus in the lower Animals. 



653. Before proceeding to consider, in more detail, the structure and 

 actions of the respiratory apparatus in Man, we may advantageously 

 glance at the mode, in which this function is effected in the lower ani- 

 mals. In the lowest and simplest, which are inhabitants of the water, 

 we do not find any special apparatus for the aeration of the fluids of the 

 body ; this being accomplished by the exposure of them to the surround- 

 ing medium, though the thin integument ; and the" interchange of the 

 layer of water (holding air in solution) in contact with the aerating sur- 

 face, is effected either by the general movements of the body, or by the 

 action of the cilia ( 234) which produce the currents necessary for this 

 purpose. Not unfrequently, the internal surfaces such as the walls of 

 the stomach and of other cavities, seem as much concerned in this 

 function as the external, or even more so ; these cavities being distended 

 with water taken in through the mouth, and this water being frequently 

 renewed by the ejection of that which has been vitiated, and by the in- 

 troduction of a fresh supply. This is the case in the Sea Anemone, 

 for example, and in many other polypes ; and there are certain higher 

 forms of the same class, in which there is a great dilatation of the 

 pharynx, which seems peculiarly destined for the aeration of the fluids, 



