Internal Workings 153 



the two openings is the opposite. How we succeeded in re- 

 versing this relationship, whereas we were unable to do any- 

 thing about the far more impractical windpipe-gullet busi- 

 ness, is something which no one has yet explained. 



CIRCULATION OF THE BLOOD 



We now come to that system which ties together all the 

 Other internal workings. It is the function of the blood to 

 carry to every living cell in the body the oxygen and the 

 food which they need for their life-processes, and to carry 

 away from them, to places where they can be disposed of, 

 the carbon dioxide and the urea and the other excretory 

 products resulting from those life-processes j and also to 

 transport to the places where they are needed the regulatory 

 hormones secreted into the blood by the ductless glands. 



In principle the system is simplicity itself: a central pump, 

 the heart, with a series of pipes to carry the blood, diminish- 

 ing in size as they recede from the heart and increasing as 

 they return. In actual construction it is very complex, only 

 slightly less so than that of the human being, with an intri- 

 cate network of thin-walled capillaries winding through the 

 tissues to bring them their blood. 



There is only one important difference between the fish's 

 circulation and the human's, but that difference is so great 

 that it accounts for many of the other differences between 

 the two animals. In the human the blood goes from the 

 heart to the lungs to be purified, and then this oxygenated 

 blood goes back to the heart to get a fresh start for its trip 

 through the rest of the body. In the fish, the blood goes 

 from the heart to the gills to be purified, and then goes on 

 through the body without any second impulse. The human 

 has two blood circuits, the fish one, as shown by Figure 22. 



The heart is a bag with muscular walls capable of rhyth- 

 mic contractions. It is the pressure of these contractions which 



