42 Hounds 



inseparable in the strict sense. There is, however, a 

 third circulation known as the portal, in other words, 

 a liver circulation of blood. After the blood has 

 circulated through the bowels, stomach, pancreas, 

 spleen, etc., it passes into the portal vein, which runs 

 into the liver, subsequently dividing into capillaries 

 in this organ, the cells of which are thus enabled to 

 store up a substance called glycogen or animal starch. 



Veins convey the blood (hepatic veins) into the 

 inferior vena cava, and this in its turn pours it into the 

 right auricle, for subsequent purification in the lungs. 



A fourth blood circulation is often spoken of. This 

 is the renal or kidney circulation of blood. 



So far no reference has been made to either the 

 structure of the blood-vessels nor yet the com- 

 position of the blood. For convenience arteries are 

 divided into three kinds, viz., s?nall, medium and 

 large, differing somewhat in their structure. In the 

 larger arteries one of the most significant features is 

 the presence of a very thick middle coat. All arteries 

 have three coats, viz., an inner, middle and outer. 



The inner coat is chiefly composed of cells, irregular 

 in their outline, and known as the endothelium, 

 which rests upon a basement membrane. 



The middle coat contains both elastic and muscle 

 fibres, arranged circularly and longitudinally. 



The outer coat consists of connective tissue and 

 elastic fibres. In the largest arteries, e.g., the aorta, 



