XII VASCULAR SYSTEM 3 I 5 



The precaudal portion of the subintestinal vein is represented by 

 a vein which runs forwards in the intestinal wall, and is one 

 of the minor affluents of the hepatic portal vein, while its 

 prehepatic section is represented in succession by the hepatic 

 vein, the heart, and the ventral aorta. Of the additional veins 

 which supplement these remnants of a primitively continuous 

 subintestinal vein, the largest and most constant are (a) the 

 posterior cardinal veins which, commencing in the kidneys and 

 receiving the blood from those organs, pass forwards to the heart ; 

 (5) a pair of anterior cardinal veins, formed by the union of 

 smaller veins from the head, including the brain, and passing 

 backwards towards the heart. At the level of the latter organ 

 each anterior cardinal vein joins the posterior cardinal of the 

 same side of the body to form a short but wide transverse vessel, 

 the Cuvierian duct or precaval vein, which opens into the hinder- 

 most of the cavities of the heart, viz. the sinus venosus ; (c) a 

 pair of inferior jugular veins by which the nutrient blood of the 

 branchial apparatus is returned to the right and left Cuvierian 

 ducts. In addition to these principal veins there may also be a 

 pair of lateral veins collecting the blood from the lateral walls of 

 the trunk, and also opening into the Cuvierian ducts : and sub- 

 clavian and femoral veins from the pectoral and pelvic fins. 



On the other hand, the primitive dorsal vessel (dorsal aorta), 

 retains not only its original position and relations, but also its 

 primary function as the main channel for the distribution of 

 arterialised blood to all parts of the body. The system of 

 lateral and probably segmentally arranged vessels, by which the 

 dorsal and subintestinal vessels were connected in the primitive 

 Vertebrata, have undergone considerable modification in all exist- 

 ing Fishes, but nevertheless retain much of their original disposi- 

 tion and relations in the pharyngeal region of the alimentary 

 canal, where they are represented by the afferent and efferent 

 vessels of the gills. 



A more detailed account of the condition of the vascular 

 system in the Cyclostomata and Fishes will now be given. 



The Venous System. The Cyclostomata, 1 as might be ex- 

 pected, exhibit a more primitive condition of the venous system 



1 J. Miiller, Vergl. Anat. d. Myxinviden, Pt. iii. (1839), Berlin, 1841, p. 186. 

 For an account of the vascular system ofBdellostoma see Jackson, Journ. Cincinnati 

 Soe. Nat. Hist. xx. 1901, p. 13. 



