At p is seen the inferior vena cava cut through, which lies in the interspace of the two great depres sors of the tail. The trunk of' the vena cava seems smaller than it really is, on account of its deep position and the overlapping of the kidney, E. As it gets beyond this part it is seen to dilate. Two veins, correSponding to the yen& iliac& of Quadrupeds, (7n, ni,) return the blood in part to the tail, and join the vena cava near the kidneys. The vein corresponding to the caudal or sacro-median of Quadrupeds is not a simple vessel, but a plexus which is surrounded and protected by the inferior spinous processes ; it is seen atf. A venous plexus from the intestinal canal (g) terminates in the right iliac vein, which is larger than the left, and thus establishes a com munication between it and the portal system. h shows a muscular vein, and i the termination of a hypogastric plexus.
The more important plexuses which commu nicate with the iliac veins are, first, the perito neal plexus (/), which in older individuals, and especially at the season of sexual excitement, is much more considerable than is here repre sented ; and secondly, the iliac or psoadic plexus (k,k), which forms an immense reser voir of venous blood. It is situated between the under surface of the depressors of the tail, which represent the psoas muscles, and the peritoneum, reaching from behind the lower extremity of the kidney to the posterior end of the abdomen, and forming a mass of closely interwoven veins, of an inch or more in thick ness, and serving to bring the subcutaneous veins of the posterior part of the body into communication with the posterior vena cava.
This plexus is fed, if we may use the ex pression, by a, an inferior vein; b, a lateral; and c, a superior vein of the tail, which unite to form an ischiadic sub-plexus, d. Laterally the iliac plexus receives from five to seven veins, which return the blood from the dorsal and lateral parietes of the abdomen, and pierce the lateral abdominal muscles to join the plexus at e,e. On its internal or mesial edge the iliac plexus communicates by many and wide aper tures with the iliac vein. At the anterior part of the abdomen the inferior cava receives the plexus phrenicus, o, o.
The, condition of the venous system above described, while it is admirably adapted to the mode and sphere of existence of the Cetaceans, presents a beautiful instance of that co-ordinate analogy to the condition of the veins in the embryo of the higher Mammals, which is ex hibited in the general form of the animals composing this the lowest order of the class.]