VEINS OF VEKTEBEATA. 



591 



and are known as the vena? vertebrales. The anterior and posterior 

 ones on either side are united, and open into the jugular vein of 

 their own side. Their connection with the left jugular disappears 

 later on, when the left vertebral veins develop transverse ana- 

 stomoses, and become connected with the right ones, and, like them, 

 open into the right jugular. 



When the cardinal veins cease to be connected with the ductus 

 Cuvieri they form prolongations of the jugular veins, which receive 

 the subclavians which come from the fore-limbs, and are known as 

 the superior vena? cava?. The vertebral veins, which collect the 

 blood from the walls of the body, are not large after the embryonic 

 period, and they are generally considerably atrophied. They cease 

 to have a paired arrangement (Ophidii), and the greater part of their 

 area is occupied by the vena cava inferior. 



We meet with similar arrangements in the Birds. A pair of jugular 

 veins, which are often unequally developed, form the chief trunks 

 for the blood returned from the anterior parts of the body. At the 

 base of the skull they are generally connected with one another by 

 a transverse trunk, into which the veins from the cervical vertebral 

 column, as well as from the head, may enter. When the left 

 jugular is atrophied, this transverse trunk forms the vessel by which 

 the blood is conveyed into the right jugular. The vertebral veins 

 are now inconsiderable vessels. The jugulars unite with the veins 

 of the anterior extremities, which form the subclavians, and the 

 trunks thus formed are again known as the superior vena? cava?. 

 As these still receive the posterior vertebral veins, a portion is 

 separated off from them, which may be seen to be derived from the 

 transverse trunks (ductus Cuvieri), which are persistent in Fishes. 

 These vena? cava?, however, open separately into the right auricle, 

 for the sinus, which is persistent in the Reptilia, here forms a portion 



Fig. 336. Relations of the great venous trunks on the heart. I Reptile 



(Python). II Bird (Sarcorhainphus). Ill Marsupial (Halrnaturus). IV Pig. 



They are all seen from behind, i Vena cava inferior, s Vena cava superior sinistra. 



d Vena cava superior dextra. ap Pulmonary artery, a Aorta, sv Sinus venosus. 



of the auricle (Fig. 336, I, sv). The vertebral veins in Birds pass 

 along a canal which is enclosed by the ribs, by which point they are 

 seen not to be the same vessels as the cardinal veins. 



