Internal Workings 155 



the division is to give the brain a supply of the very best 

 blood, for in both fish and human, one stream goes directly 

 to the head and then, without being called on to do any 

 further work, is returned to the heart for repurification. 

 (This is not shown in the sketch in order not to complicate 

 the diagram.) The other stream goes to all the rest of the 

 organs. It goes to the muscles of the fins and tailj it goes 

 through the kidneys in a network of capillaries known as 

 the renal-portal system, where it disposes of some of the 

 waste products of which it has relieved the other tissues j it 

 passes around the digestive tract, where it absorbs nourish- 

 ment j it goes through the liver in another capillary net- 

 work called the hepatic portal, where it stores some of the 

 excess nourishment, and it finally returns to the heart. 



The fish's blood, like man's, is red in appearance because 

 of red blood corpuscles, and these corpuscles, as in man, con- 

 tain that substance hemoglobin, which is important because 

 of its ability to attract, absorb, and transport comparatively 

 large quantities of oxygen. Compared to man's, the fish's 

 blood is thick, viscous. It does not flow as readily as man's 

 does. 



Now let us consider the combined results of all these char- 

 acteristics of the fish's circulatory system. In the first place, 

 because of the long circuit which the blood has to make on 

 the impulsion of only one series of heart chambers, it is under 

 much less pressure than is human blood. Cut a man's artery 

 and the blood spurts outj cut a fish's artery and it just 

 dribbles out. Because of this low pressure, and also because 

 of its viscosity, the fish's blood does not flow so fast through 

 its vessels. Because of this comparatively slow flow, and also 

 because water has less oxygen to yield than has air, the oxy- 

 genation of the fish's blood is less than that of man's. But 

 low oxygenation means low temperature j and the cooling 

 of the blood by the surrounding water as it flows through 



