vi VENOUS SYSTEM 425 



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In the head legion the anterior cardinal becomes in great part 

 replaced by a lateral cephalic vein in a manner similar to that 

 already described for Lepidosiren. 



Birds. — -In Birds the development of the venous system pursues, 

 as we should expect, a similar course to that already described for 

 Beptiles. Amongst the differences in detail the most striking is 

 that the two vitelline veins become completely fused into a single 

 vessel, the ductus venosus, through the hepatic region, before there 

 are any signs of a hepatic network. This may be regarded as a 

 backward extension of the fusion of the two vitelline veins which 

 gives rise to the heart. The ductus venosus secondarily becomes 

 surrounded by the liver rudiment and a network of channels spreads 

 out from it in the liver substance. The allantoic veins behave as 

 in Lacerta except that a small vestige of the leit is said to persist 

 throughout life. 



The reduction of the tail region in modern birds has brought 

 with it a modification of the caudal vein which is here paired, taking, 

 the form of a simple backward prolongation of the posterior cardinal. 

 The main channel of the posterior cardinal runs along the outer 

 edge of the opisthonephros but later on a slender channel appears along 

 its inner edge — that on the right side being continuous with the 

 posterior vena cava of which it forms simply a backward prolonga- 

 tion. The two inner channels undergo fusion so that the blood 

 from the kidneys can drain away entirely into the posterior vena 

 cava and this is followed as in other cases by the atrophy of the 

 portion of posterior cardinal lying in front of the opisthonephros. 

 This atrophy extends as far forwards as the subclavian vein which 

 1 in the Fowl opens into the posterior cardinal vein some distance 

 from its front end. The portion of posterior cardinal lying in front 

 of this point is consequently saved from disappearance and persists 

 as a portion of the definitive subclavian vein of the adult. It will be 

 understood that the blood of the outer channel of the posterior 

 cardinal, which reaches it from the caudal vein, from the posterior 

 limb, and from the body-wall, passes entirely through the opistho- 

 nephric network towards the posterior vena cava, in other words that 

 there is at this time a typical renal portal system. 



As the metauephros develops, its tubules are also mixed up with 

 the sinuses connecting external and internal channels of the posterior 

 cardinals, so that it too has for a time a functional renal portal system. 

 Later on however one of the channels through the metanephros 

 becomes enlarged and the blood-stream passes directly through it to 

 the posterior vena cava without traversing the meshes of the network. 

 A true renal portal system then no longer exists and the reason for 

 its disappearance is no doubt to be found in the fact that the vascular 

 network of the kidney has become connected with the arterial system. 

 Obviously this will give a much more efficient circulation than the 

 original one, owing to the higher blood pressure in the dorsal aorta 

 and renal arteries than in the renal portal veins which have the 



