XII VASCULAR SYSTEM ys 
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; 
(b) 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,’ as might be ex- 
pected, exhibit a more primitive condition of the venous system 
1 J. Miller, Vergl. Anat. d. Myxinoiden, Pt. iii. (1839), Berlin, 1841, p. 186. 
For an account of the vascular system of Bdellostoma see Jackson, Journ. Cincinnati 
Soc. Nat. Hist. xx. 1901, p. 13. 
