VEINS OF VERTEBEATA. 593 



coronary vein, wliile the valvula Thebesii, whicli is placed at its 

 opening into tlie riglit auricle, forms, for a long time, the valve of 

 the left superior vena cava. The right superior cava is now the 

 sole anterior trunk (Cetacea, Carnivora, Primates). 



When the trunk of the left superior vena cava is reduced, the 

 cardinal veins, or the vertebral veins developed in their area, undergo 

 great changes. Whde in the first case they open into the vena cava of 

 their own side (A), and while in the second they pass separately into 

 the left side of the right auricle, owing to the development of a right 

 vena cava (B); when the vascular passage which leads directly to the 

 heart is reduced, they become connected with the right vertebral 

 vein. The left vertebral vein is connected with the right one by 

 transverse anastomoses, and when the connection between its upper 

 end and the left superior cava is broken, it is converted into a 

 vena hemiazygos, while the right one, which still retains its 

 primitive position, becomes the vena azygos (Fig. 339). When 

 the two superior cavae persist, the two vertebral veins are not, in all 

 cases, unchanged; one trunk often becomes much larger than the 

 other, which may even be so far reduced as to disappear altogether. 

 In this case a vena azygos receives the intercostal veins of both 

 sides ; and this sometimes opens into the left, and sometimes into 

 the right superior vena cava, or even into the only one that is 

 present, as, for example, in the Carnivora (Fig. 337, G, az). 



In most Mammals the roots of the jugulars are formed of a 

 number of veins from the external and internal cephalic regions, 

 one of which conducts a part of the blood from the cranial cavity 

 through the jugular foramen. It forms a small vessel only, for the 

 greater part of this blood passes out in a canal (canalis temporalis), 

 which is either placed between the petrosal and squamosal, or in the 

 latter only. When the foramen jugulare is enlarged, the vein, which 

 in other cases is a small one, increases in size, and gradually becomes 

 the most important of all the vessels which come from the skull ; in 

 this case it forms, as it does in the Primates, the internal jugular 

 vein. The other veins gradually unite to form the external jugular, 

 which is the most important one in most Mammals. 



§ 440. 



The second large venous tract is very small in Fishes, for in them 

 it is merely represented by the hepatic veins, which are united into 

 one or more trunks, and open into the common venous sinus. When 

 the tract of the cardinal veins is diminished in extent, a new tract is 

 formed in connection wdth the hepatic veins — that of the inferior 

 vena cava (Amphibia). This venous trunk collects blood from the 

 kidneys, and is, therefore, the vena renalis revehens (Fig. 338, A, ci). 

 The blood from the hinder extremities passes into an iliac vein 

 {A, i), which, in the Urodela, receives on either side a branch of the 

 divided caudal vein. It breaks up in the liver, and forms a vena 

 renalis advehens. A branch of the iliac vein passes to the middle 



