viii THE FUNCTIONS OF THE BODY 155 



physiolog}^. Light has been thrown on the fact of 

 physiological correlation, in which different parts work 

 into one another's hands in a remarkably harmonious 

 and adaptive manner, for the internal secretions are 

 carried from organ to organ. Thus from the reproductive 

 organs as they mature influences pass to distant parts 

 of the body. In subtle ways the womb is prepared for 

 the development of the offspring and the milk-glands 

 for the day of its birth. 



9. Functions of the Blood. Many animals get on well 

 without blood, but when these cases are examined it 

 will be found that there are internal arrangements for 

 keeping up currents of water or of nutritive fluid. A 

 sponge often grows to be a large animal, but it is tra- 

 versed by water-ways along which food-supplies and 

 oxygen are swept in and waste-products swept out. 

 Again, in an animal like the common jellyfish (Aurelia 

 aurita), so often stranded on flat beaches, we can see that 

 eight branched and eight unbranched canals radiate out 

 from the central stomach and lead into a circumference 

 canal round the margin of the disc. Thus the food in 

 the alimentary system is carried everywhere. To take 

 another example, the liver-fluke (Distomum hepaticum), 

 which lives in the liver of the sheep (see p. 234), feeds 

 on the blood of its host, but has no blood of its own. 

 But it has a food-canal with extraordinarily complicated 

 branching, which transports the nutriment everywhere, 

 and still more branched is an excretory system which 

 drains the leaf -like body of its nitrogenous waste. But 

 while blood can be dispensed with, the advantages of 

 its presence are obvious. 



In a higher animal the chief functions of the blood are 

 four. (1) It is a carrier of the digested food-stuffs. (2) 

 In the arteries and pulmonary veins it is a carrier of 

 oxygen (loosely united with the haemoglobin of the red 

 blood corpuscles) ; in the veins and pulmonary arteries 

 it is a carrier of carbon dioxide (loosely united with some 

 constituents of the fluid or plasma). (3) It is a carrier 

 of nitrogenous waste products. (4) It is a distributor 

 of the hormones or internal secretions. In addition to 



