60 ANIMAL BIOLOGY. [Part I. 



and (2) a pump, the ventricle. Between the two in each case 

 are valves, which permit the blood to flow from the receiver into 

 the pump, but prevent its flowing from the pump into the 

 receiver. 



In the course of the circulation, beside the senso-motor arc, are 

 (1) lungs, (2) kidneys, (3) the alimentary canal. In the lungs 

 the red corpuscles are freighted with oxygen, which they convey 

 to various parts of the body, returning to the lungs empty. The 

 blood which contains the laden corpuscles, and which is bright- 

 red in colour, is called arterial ; that which contains the 

 exhausted corpuscles, and which is dull-red in colour, is called 

 venous blood. In the lungs also the plasma delivers up one 

 main product of tissue waste, the carbonic acid gas. So that the 

 lungs have a double function. In their relation to the blood 

 corpuscles they have the function of arterialisation : in their re- 

 lation to the blood plasma they have an excretory function. In 

 the kidneys a further and special product of animal metabolism 

 (see p. x), the urea, is eliminated from the plasma. In the ali- 

 mentary canal the plasma is enriched with nutritive material. 



In each of these organs the blood-vessels branch and sub- 

 divide until they form a close meshwork of minute vessels 

 termed a capillary plexus. In each of the multitude of capillaries 

 which make up such a plexus the walls are so thin that tissue 

 nutriment and tissue waste is readily given up or received by 

 the blood ; and from the extreme fineness and the great number 

 of the tubes, the amount of surface thus bathed by the blood is 

 relatively enormous. The sub-division of arteries into capillaries, 

 and the union of capillaries into veins, may readily be observed 

 under the microscope in the web of a frog's foot. The frog must 

 be lightly tied to a piece of thin board or thick card-board, in one 

 edge of which a V-shaped notch has been cut. Over the- notch 

 the web is placed, threads being tied to two of the toes to keep 

 them sufficiently apart. The frog should be kept moist by 

 placing a piece of wet blotting-paper over his body. In every 

 organ of the body the blood-vessels form such capillary plexuses 

 as are thus seen spread out like a network in the web of a frog's 

 foot. 



