STRUCTURE OF THE VERTEBRATES 239 



from the dorsal aorta which are absent in the lower groups, or 

 arise as branches from other vessels. The genital arteries are 

 paired and supply the reproductive glands. The adrenal arteries 

 go to the adrenal glands which are located near the kidneys; 

 and the renal arteries go to the kidneys. The latter are present 

 in the dogfish, but arise as very small, metameric vessels. In 

 the amniotes these are collected into a few vessels, or a single 

 pair as in the mammals. 



6. The Systemic Veins. The systemic veins are those which 

 enter directly into the heart without breaking into capillaries. 

 Of these systemic vessels, the anterior cardinals remain rela- 

 tively unchanged. In the higher vertebrates they are known as 

 the internal and external jugular veins. The internal pair drains 

 the brain and upper portion of the head, the external pair 

 drains the superficial musculature. In the mammal there are 

 frequent anastomoses between the two veins, and their inde- 

 pendent origin can be less clearly seen. 



The lateral veins of the dogfish, which drain the sides of the 

 body and the paired fins, disappear in the urodeles. However, the 

 abdominal vein arises as a pair of veins which fuse, and this 

 median vessel is considered homologous with the laterals. The 

 alligators have two lateral veins. The other reptiles have an 

 abdominal vein similar to that of the urodeles. In the mammals 

 the lateral (or abdominal) veins practically disappear, and the 

 subclavian veins join the jugulars before entering the heart. 



The postcardinal veins are very prominent in the dogfish, 

 passing anteriorly from the kidneys, and joining the anterior 

 cardinals before entering the heart. The further evolution of 

 these veins is intimately correlated with the development of the 

 postcava, or posterior vena cava, and these two veins must be 

 considered together. The postcava makes its first appearance in 

 the dipnoan fish. In the urodele amphibia it is a large vessel 

 passing from the anterior level of the kidneys to the right 

 auricle. At the kidneys the vessel divides and passes to each 

 kidney, taking over much of tlie function of the cardinals. The 

 latter are small vessels which appear to be branches of the 

 postcava, and pass anteriorly to enter the ducts of Cuvier, 

 draining the intercostal muscles and thorax. Embryological de- 

 velopment shows that the postcava grows posteriorly from the 



