[Illustration: 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 {319}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). Characteristic features in the venous system of _Mustelus_, as also of Elasmobranchs in general, are the development of transverse connexions between certain of the principal paired veins, and the tendency of many of the main veins to enlarge into more or less irregularly-shaped sinuses. In its broad outlines the venous system of the Teleostomi agrees with that of Elasmobranchs, but is nevertheless characterised by several more or less important modifications, while at the same time exhibiting many differences in minor details.