628 MR. c. IT. o'donoghue on the 



veins and runs backwards in the mesenteiy to the kidneys. 

 Between them it breaks up into two branches, which run pos- 

 teriorly along their mesial borders. These branches meet the 

 kidneys at a point some distance from their front end, and there 

 each receives a branch from the anterior part of the kidneys. 

 Subsequently the parts of the posterior cardinal veins in front of 

 each mesonephros disappear, and so all the blood from the caudal 

 veins has to pass through those organs. 



Still later the adult kidney arises caudally to the mesonephros, 

 and then the end pai-t of each posterior cardinal vein comes to lie 

 on its ventral and external border, while a continuation of one of 

 the posterior mesonephric branches of the post-caval vein extends 

 along the inner border of the permanent kidney. 



The left umbilical vein loses its opening into the ductus Cuvieri, 

 and all its blood is taken to the liver network. The right 

 umbilical vein, on the other hand, retains its original opening 

 into the ductus for some time (text-fig. 90). In the subsequent 

 changes the post-caval vein increases in size, and the part of the 

 omphalo-mesenteric vein joining it breaks up into a venous net- 

 work with the caudal extension of the liver. As the two umbilical 

 veins are joined by an anastomosis at the navel, it is possible for 

 the right to disappear, which it does, leaving the left, which, 

 however, disappears soon after birth *. Ultimately the omphalo- 

 mesenteric vein, lying on the dorsal side of the liver, can only 

 communicate with the sinus venosus via the hepatic network and 

 the post-caval vein, which is on the ventral side of the liver. 



The anterior cardinal veins originate in a similar way to those 

 of Selachians and Amphibians, but the parts of these veins in the 

 head are completely replaced in an interesting way (vide Grosser 

 and Brezina, 19). The original cardinal vein runs backwards 

 from the infraorbital and the anterior cerebral veins ventrally to 

 the cranial nerves into the ductus Cuvieri. Three venous rings 

 ai^e now formed in succession ; the first around the root of the 

 facial nerve and the auditory vesicle, the second around the root 

 of the glossopharyngeal nerve, and the third around the vagus 

 root. Their median portions afterwards disappear, and the external 

 ones unite to form one lateral trunk. In the meantime two new 

 vessels come to open into the anterior cardinal : one, the median 

 cerebral vein, coming from the cerebellum, opens just posterior to 

 the trigeminal nerve ; and the other, the posterior cerebral vein, 

 coming from the medulla, opens posterior to the vagus, leaving 

 the skull by the foramen magnum. 



Still later the lateral trunk opens anteriorly into the infi-a- 

 orbital vein by means of an extension by the side of the second 

 and third branches of the trigeminal nerve, and posteriori}'- past 

 the hypoglossus into the anterior cardinal opposite to the pos- 

 terior cerebral vein. In this way is formed a new, complete 



* Generally the umbilical vein disappears in the adult snake, but remains of it 

 persist in some species, e. g. Boa constrictor, B. diviniloqua, Python regius, Eunectes 

 murinus, Coralhis cookii (Beddard, 4 & 5). 



