38 CLASSIFICATION 



several other ways, e.g. it prevents the digesting food from be- 

 coming putrid. 



The digested foods are now absorbed into the minute blood- 

 vessels which branch in the mucous membrane, the digested fats, 

 however, passing into other minute tubes called lacteals, which 

 ultimately communicate with the blood-system. It must, of course, 

 be understood that blood-vessels and lacteals do not open into the 

 cavity of the gut, but the digested materials, so to speak, soak 

 into them. Those parts of the food which are not digested pass 

 out of the body altogether. 



CIRCULATORY ORGANS 



Branching elaborately within the body, and sending its 

 twigs into almost all its different parts, is a closed set of tubes 

 constituting the blood-system. Propelled by the heart or central 

 pump the red fluid known as blood is constantly circulated 

 through this system. Supplementary to the blood-system we 

 have a set of spaces and tubes (of which the lacteals mentioned 

 above form a part) making up the lymph-system. This contains 

 a pale fluid, the lymph, which it pours into certain of the great 

 blood-tubes. The two fluids, blood and lymph, especially the 

 former, may be regarded as a sort of medium of exchange as 

 regards the various parts of the body. Certain things are taken 

 out of them to meet local requirements, and certain other things 

 pass into them; 



Speaking more precisely, the functions of the circulatory 

 system are as follows: (i) It distributes the digested food, i.e. 

 the materials for repairing waste and providing for growth; 

 (2) It removes waste matters from all parts of the body, and carries 

 these to organs which can get rid of them; (3) It carries oxygen 

 to promote this waste; and (4) It distributes heat and equalizes 

 the temperature of different parts. 



THE BLOOD (figs. 14 and 15). When examined under the com- 

 pound microscope a drop of blood is found to consist of colourless 

 liquid (plasma) in which float a vast number of minute bodies, the 

 blood- corpuscles. These are of two kinds red, and colourless or 

 white. The former are biconcave discs not more than ^3200 of an 

 inch in diameter, and they owe their colour to the presence of a 

 peculiar pigment (haemoglobin) that, as we shall presently see, 

 is of great importance in respiration. The white or colourless 



