6 ZOOLOGY SECT. 



ailed hmmoglohin, which has a strong affinity for oxygen ; and the 

 xygen from the air, when it enters the blood, enters into a state 

 f loose chemical combination with it. In this state, or simply 

 issolved in the fluid plasma of the blood, the oxygen is conveyed 

 hroughout the body. 



Thus the blood, besides receiving the solid and liquid food from 

 he alimentary canal and carrying it throughout the body for 

 istribution, receives also the oxygen or gaseous food, and _ supplies 

 t to the parts requiring it. In all parts of the body in which 

 ital action is taking place chemical changes are constantly going 

 n. These chemical changes in the tissues, having for their result 

 he production of heat, motion, secretion, and nerve-action, are 

 or the most part of the nature of oxidations, and involve a constant 

 onsumption of oxygen ; while a product which becomes formed 

 s a result of this action is carhonic acid gas. 



To carry out all the functions which it has to perform as a 

 listributor of nourishment and oxygen and a remover of carbonic 

 icid, the blood has to be moved about through the vessels — to 

 irculatc throughout the various organs. In the lowest forms in 

 vhich a definite blood-system is to be recognised, this movement 

 s effected in great measure by the general movements of the 

 )ody of the animal. In others certain of the vessels contract and 

 Irive the blood through the system ; such contractions are of a 

 leristaltic character, the contractions being of the nature of con- 

 itrictions running in a definite direction along the course of the 

 'essel, with an effect similar to that produced by drawing the 

 land along a compressible india-rubber tube. 



In all higher forms the movement of the blood is effected by 

 neans of a special organ — the heart. The heart is a muscular 

 )rgan which by its contractions forces the blood through the 

 system of vessels. In its simplest form it usually consists of two 

 ihambers, both with muscular walls, — the one, called the auriele, 

 •eceiving the blood and driving it into the other, which is called 

 ihe ventricle. The latter, in turn, when it contracts, drives the blood 

 through the vessels to the various parts of the body — the return 

 )f the blood backwards to the auricle from the ventricle being 

 prevented by the presence of certain valves, which act like folding 

 loors opening from the auricle towards the ventricle, but closing 

 ivhen pressure is exerted in the opposite direction. In the higher 

 mimals the heart becomes a more complex organ than this, with a 

 arger number of chambers and a more elaborate system of valves. 



Carbonic acid, as already mentioned, is a waste-product con- 

 stantly being produced in the tissues and being carried off by the 

 jlood to pass out by the gills or lungs. Besides the carbonic 

 icid, there are constantly being formed waste-substances of another 

 ;lass — viz., substances containing nitrogen, of which uric acid and 

 'irea are the principal ultimate forms. These are separated from 



