In the second modification, the veins which return from the posterior part of the body are received into this separate system: The caudal vein, which brings back the blood from the skin and muscles of the posterior part of the body, divides into two branches, which, having received some veins returning from the middle part of the body, pass to the kidneys of each side, and distribute their branches in the substance of these glands.
In the third modification, the veins of this system are formed in the same manner as in the preceding, excepting that the caudal, or other vein returning from the posterior part of the body, gives off a branch to the vena portce.
The blood, returning from the middle and posterior part of the body in the first and second modification of this system, is conveyed only to the kidneys ; but in the third it is divided between the kidneys and the liver. The inferior vena cava of the common venous system, in the second and third modification is composed of the veins returning from the kidneys and testicles or ovaries. In the first modification, the caudal vein receives the veins returning from the kidney's, is united with the veins of the testicles or ovaries, and in this manner forms the inferior vena cava.
The above is a general description of this peculiar system of veins. With reference to its existence in fishes, it will suffice to add, that in different genera this venous system appears in all its modifications.
In Reptiles, the kidneys are generally situ ated very far back, even within the cavity of the pelvis, where a sacrum exists, as in the Chelonian and Saurian orders ; and in these tribes they are very partially covered by- the peritoneum being firmly imbedded in the sacral region. But in serpents, in conse quence of the elongated form of the body, and the complete flexibility of every portion of the spine, the kidneys are peculiar both in their position and general structure. The kidneys of an Ophidian are not placed upon the same level, but the right is situated much more anteriorly than the left ; a circumstance which much facilitates the packing of the abdominal viscera, and contributes greatly to ensurg the free rnovements of the vertebral coluinn at this place. For the same reason the kidneys of a serpent are divided into numerous lobes of a compressed reniform shape, placed in a longitudinal series upon the external surface of the ureter, and loosely connected to each other and to the spine by cellular tissue and a fold of the peritoneum.* The kidneys of
reptiles, like those of fishes, have no distinction of cortical and medullary substance ; and the urinary tubules pass immediately from the substance of the kidney into the ureter.
The peculiar venous system described by Prof. Jacobson is found in reptiles under the form of the third modification ; that . . . .
namely, In wmcn the blood returned by thE veins from the back and the posterior part of the body, is divided between the kidneys and the vena portm. The exact distribution of these veins in the substance of the kidney of the Boa has been clearly demonstrated by Mr. Bowman, as will be shown, when we come to speak of the minute anatorny of the kidney. The arterial branches, which are comparatively of small size, are derived from the abdominal aorta.
In Birds the kidneys are elongated in form, commencing immediately below the lungs, and extending symmetrically on each side of the spine, as far as the termination of the rectum, The posterior surface is moulded into the cavi. ties formed by the bones on which it rests, The ureter proceeds from the anterior aspect, the secreting tubules passing immediately, as in fishes and reptiles, into the ureter, so that there is no pelvic cavity in the organ, nor any division into cortical and medullary substance. The outward form of the kidney is very various, and the surface is divided in different species into a variable number of lobes. Each kidney is invested by a delicate capsule, which extends into the substance of the gland be tween its lobular divisions. Their texture is much more friable than in mammalia, readily yielding under the pressure of the finger, to which they give a granular sensation as their substance is torn asunder.* The peculiar venous system already referred to is described by Jacobson as existing in birds under the form of the third modifica tion. But the arrangement of the veins differs from that observed in reptiles in this respect, that the crural vein after giving off a superior branch to the superior lobe of the kidney, and an inferior to the ischiatic vein, sends a middle branch direct to the vena cava. All the blood, therefore, which in birds returns from the posterior part of the body, is carried partly to the kidneys, partly to the portal vein, and partly, but in small quantity, is con veyed in a direct manner to the vena cava. There are no regular emulgent arteries in birds, the kidneys deriving their arterial blood from various branches of the abdominal aorta.