The precaudal section of the primitive subintestinal vein, now termed the internal intestinal vein (Figs. 186 and 187), traverses the spiral valve as it passes forwards to the liver, but from a physiological point of view is now merely one of the factors of the great hepatic portal vein, the principal tributaries of which are the veins from the stomach and intestine, including the rectal gland, and the pancreas and spleen. On entering the liver the hepatic portal vein divides into two principal branches for the right and left halves of the gland. From the liver the blood is conveyed by two hepatic veins to the sinus venosus.
Fig. 186.—Venous system of Mustelus antarcticus. a, Auricle; a.c, anterior cardinal; b.a, conus arteriosus; br.v, brachial vein; c.d, Cuvierian duct or precaval vein; c.v, caudal vein; cl.v, cloacal vein; f.v, femoral vein; h.s, hyoidean sinus; h.v, hepatic vein; i.i.v, internal intestinal vein; i.j, inferior jugular; k, kidney; l, liver; l.v, lateral vein; md.v, mandibular vein; n.h.v, nutrient hyoidean veins; o.s, orbital sinus; p.c, posterior cardinal; p.v, hepatic portal vein; rp.v, renal portal vein; sc.v, subscapular vein; sp.v, spermatic vein; s.v, sinus venosus; v, ventricle; v.a, ventral aorta. (After T. J. Parker.)
The lateral veins (Fig. 186) are situated in the lateral walls of the abdomen, immediately external to the peritoneum (Fig. 187). Each vein begins near the pelvic fin, where it is connected with its fellow across the dorsal face of the ischio-pubic cartilage, and thence runs forward towards the pectoral fin. At its origin the lateral vein receives a femoral vein from the pelvic fin and a cloacal vein, and also, near its anterior end, a brachial vein from the pectoral fin, finally joining the Cuvierian duct of its side.[[361]]
Fig. 187.—Diagrammatic transverse section of an Elasmobranch, showing the position of the principal longitudinal blood-vessels. c, Coelom; d.a, dorsal aorta; d.c.v, dorsal cutaneous vein; d.i.v, dorsal intestinal vein; i, intestine; i.i.v, internal intestinal vein; l.c.v, lateral cutaneous vein; l.v, lateral vein; m.v.a, myelonic vein and artery; p.c.v, posterior cardinal vein; sp.c, spinal cord; sp.v, spiral valve; v, vertebral centrum; v.c.v, ventral cutaneous vein; v.i.v, ventral intestinal vein. (From T. J. Parker.)
The anterior cardinal vein is situated directly above the gill-arches of its side of the head, and extends forwards from its junction behind with the Cuvierian duct to the outer side of the auditory capsule, where it communicates by a valvular orifice with a large sinus surrounding the eye-muscles (orbital sinus), and ventrally, by means of a similar aperture, with another large sinus, the hyoidean sinus, which lies on the outer face of the corresponding hyoid arch, and is continuous ventrally with its fellow of the opposite side. Into the orbital sinus open the anterior facial vein from the anterior and external regions of the head, and the anterior cerebral vein from the lateral half of the brain, and, into the hyoidean sinus, the nutrient veins from the hyoidean hemibranch.
The inferior jugular veins are situated beneath the branchial apparatus. Each vein begins anteriorly by communicating with the hyoidean sinus of its side, and, after receiving the nutrient veins from the holobranchs of the first four branchial arches, opens into the corresponding Cuvierian duct.
The venous blood from the heart itself is collected into two coronary veins, which open into the sinus venosus.
In addition to the more important veins already described, there is also a series of median and lateral cutaneous veins communicating at different points with certain of the more deeply seated veins (Fig. 187).