1 52 VERTEBRATE ZOOLOGY 



The Vascular System. This is made up of the following or- 

 gans: (i) the heart; (2) the arteries, the vessels through which 

 the blood is carried away from the heart; (3) the veins, the 

 vessels through which the blood returns to the heart; and 

 (4) the capillaries, the minute vessels connecting the veins and 

 arteries. 



The heart and the great veins and arteries which are in direct 

 connection with it have already been studied. We shall now make 

 a detailed study of the veins, and after them of the arteries. 1 



The veins may be divided into two groups: (1) the systemic 

 veins, those which bring venous blood from the various organs and 

 tissues to the sinus venosus, with their branches ; and (2) the pul- 

 monary veins, which enter the left auricle, bringing arterial blood 

 from the lungs. 



The veins of the first group may be further subdivided into 

 (a) the caval veins and their branches, which bring blood directly 

 to the sinus venosus ; and (b) the portal veins, which carry it first 

 to the liver and kidneys. 



The Portal Veins. Two systems of portal veins are present : the 

 hepatic portal veins, by which blood is carried from the stomach, 

 intestine, and spleen to the liver ; and the renal portal veins, which 

 carry the blood of the hind quarters to the kidneys. The renal 

 portal system is not, however, complete in turtles ; for some of its 

 branches pass directly through the kidneys to the postcaval vein 

 and thence to the heart, instead of breaking up into capillaries in 

 the substance of the kidneys. The beginning of a retrogressive 

 process in the renal portal system is thus indicated, which is car- 

 ried still further in birds and completed in mammals, in which 

 there is no such system. The two portal systems in turtles are 

 joined by the two abdominal veins which, as we have already seen, 

 lie in the ventral abdominal wall and which are very conspicuous 

 when the plastron is removed. 



1 Both the veins and the arteries can be studied to advantage in the turtle with- 

 out being injected ; this is especially true of the veins, which are colored dark by 

 the blood which remains in them after the death of the animal. If the veins are 

 injected, however, it must be done at several places, — in the portal or one of the 

 abdominal veins and in the postcaval. The entire arterial system is easily in- 

 jected through the ventricle of the heart. 



