THE VASCULAR SYSTEM 227 



triangulare in approximately the middle line. It then turns lateral- 

 wards, dorsal to the M. genio-hyoideus and ventral to the M. rectus 

 cervicis to join the thyroid vein. It is very variable in size and 

 extent. 



After receiving the above-mentioned vessel the thyroid vein con- 

 tinues posteriorly as the V. revehens (Druner), ventral to the aortic 

 arches, to join the lingual vein. It occasionally unites with the facial 

 vein just before this vessel joins the lingual. 



It is not difficult to see how the system of the external jugular has 

 been derived from the sinuses of the fish.^ Thus the superior and 

 inferior palpebral veins draining the peri-orbital network, and the 

 superior and inferior facial veins appear to be derived, in part at least, 

 from the orbital, post-orbital, and hyoidean sinuses of the Selachian, 

 while the lingual and thyroid veins represent the inferior jugular 

 sinus. 



Veins of the Trunk and Viscera (Pis. XV and XXII). 



A. The post-caval (v.c.p.) vein is a single, unpaired, more or less 

 median vessel which first originates among the Dipnoi, and takes over 

 the function of the post-cardinals by returning the blood from the 

 posterior region of the body to the heart. It is general among the 

 Amphibia and all higher groups. In the Salamander the transference 

 of function is not complete, since the post-cardinal veins persist in 

 the adult and are quite functional. In a few cases the post-caval may 

 be wanting, with a consequent enlargement of one or other of the 

 post-cardinal veins. These cases throw considerable light on the 

 probable mode of origin of the post-caval, but the question will be 

 reconsidered after giving a description of the vein as it is normally 

 found in the adult Salamander. 



The post-caval vein arises near the posterior end of the kidneys 

 by the fusion in the middle line of a fairly large renal vein from either 

 kidney. It continues anteriorly, ventral to the dorsal aorta, to the 

 level of the coeliaco-mesenteric artery, receiving during its course 

 numerous renal factors from the kidneys and two or three genital 

 (spermatic or ovarian) factors from the left gonad and fat-body. The 

 vein now divides: one half passes dorsally and almost immediately 

 divides again as the post-cardinal veins, while the other, turning some- 

 what to the right, passes antero-ventrally into the liver as the true post- 

 caval. The genital factors from the right gonad enter at about this 

 point, either into the post-caval, or into the stem which is combined 

 as the post-cardinals. The post-caval enters what appears to be 



I Cf. 0'Donoghue(i9l4). ^.... 



