24 ZOOLOGY. 



vein, to distinguish them from the branches of the por- 

 tal vein which convey the blood to the capillaries of the 

 liver, and in which the blood is flowing from larger vessels 

 to smaller. (On this nomenclature, it is evident that 

 arteries have branches, while veins have factors, the portal 

 vein alone having both.) The liver -capillaries also receive 

 blood from an artery, as we shall see, and the mixed blood 

 after undergoing certain important changes passes on into 

 the factors of the hepatic vein and so into the general 

 system of veins leading heartwards. The function of this 

 portal system appears to be this the blood coming from 

 the capillaries of the gut contains, as we have seen (chap, ii., 

 13), much material absorbed from the chyme, especially 

 sugar and proteids. The amount of these varies, however, 

 according to the feeding or fasting of the rabbit and the 

 particular food it eats. Now it is important that the blood 

 should contain its constituent compounds in fairly constant 

 proportions : this is secured by the metabolic action of the 

 liver-tissue, which withdraws from the blood-plasma the 

 excess of sugar (and possibly proteids also) and stores it in 

 the form of an insoluble carbohydrate called glycogen, and 

 restores it to the blood again in well-regulated amounts. 

 It acts, in fact, like a warehouse in which imported produce 

 is stored instead of being allowed to flood the market as 

 soon as it arrives. This, however, is a sample of the liver's 

 functions, not a complete account. 



8. The Systemic Veins. The general arrangement of 

 the systemic veins (i.e. all except the portal and pulmo- 

 nary) is shown in fig. 4, the multitudinous minor factors 

 being of course omitted. It will be seen that they are on the 

 whole fairly symmetrical right and left, though not per- 

 Tectly so. Entering the dorsal side of the heart are three 

 large vence cavce or caval veins a right and left f precaval 

 (vena cava superior or anterior) and a single postcaval 

 (vena cava inferior or posterior.) The former have each as 

 factors a small internal jugular and large external jugular 

 from neck and head, a subcfavian from shoulder and fore- 

 limb, mammary and intercostal veins from thorax- walls, and 

 sundry minor ones. A. small azygos vein, collecting blood 



