8 COMPARATIVE ANATOMY 



the blood and those contained in the air (or, in the case of 

 aquatic animals, air dissolved in water). In ourselves the 

 respiratory mechanism consists of a pair of sacs, the lungs, 

 with thin walls in which exceedingly fine blood-vessels ramify. 

 By means of special muscular arrangements the lungs are 

 alternately expanded and compressed like bellows ; air rushes 

 in and out, and in the cavities of the lungs it is separated by 

 such very fine membranes from the blood that exchange of 

 gases readily takes place. The blood absorbs oxygen from 

 the air and carries it to the tissues ; the oxygen combines with 

 tissue materials : these, being oxidised, break down into simpler 

 substances and liberate energy. The simpler substances must 

 be got rid of, or they would accumulate and clog the machine. 

 The blood is again the agent for carrying away waste matter. 

 The principal waste matters are carbonic acid, water, and nitro- 

 genous bodies — e.g. urea. Carbonic acid, a gas, is got rid of 

 by the lungs, water by the lungs, the skin and the kidneys, the 

 nitrogenous bodies by the kidneys. Thus we find that there 

 are special mechanisms for getting rid of waste matters, or 

 excreta, as they are called, and we recognise the skin and 

 kidneys as excretory ; the lungs are in part excretory, but 

 they also serve for introducing oxygen into the system. 



Thus there is a continual exchange of material going on in 

 the animal body. Fresh material is coming in, waste material 

 is going out, and the blood is the medium of exchange. It 

 carries the fuel to the places where it is wanted, and removes 

 the ashes ; it also carries the torch, in the shape of oxygen, 

 which kindles the flame and burns up the material. The whole 

 series of exchanges is expressed by the word metabolism. 

 We can distinguish two phases of metabolism, the building- 

 up processes from food to living matter, called anabolism ; 

 the breaking - down processes, with formation of carbonic 

 acid, water, and urea, called katabolism. The several organs 

 which we have spoken of are not always in action, nor do they 

 act at haphazard. On the contrary, they perform their functions 

 when and as they are required, and that in a definite order 

 and sequence. They work harmoniously, not each organ for 

 its own ends, but all combining for the good of the whole 

 individual of which they form the parts. This co-ordination 

 of functions is effected by the nervous system, which may be 

 likened to a great telegraph system with a central office from 



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