THE VEINS
The two cranial venae cavae (Figure 37—16/7) are satellite to the brachiocephalic arteries and receive tributaries (jugular and subclavian veins) from the neck and head and the breast and wing.
The right jugular vein, always larger than the left, is visible through the skin and available for venipuncture (Figure 37-15). However, this is not possible in pigeons, in which the skin is very thick in this area. Venipuncture in these birds is done from the medial metatarsal vein. In many small cage birds the left jugular is very small. The cutaneous ulnar vein (wing vein), subcutaneous on the ventral surface of the extended wing, may also be used for the administration of fluids or collection of very small volumes of blood (Figure 37-13/9). The habit of clipping a claw for a small amount of blood is now condemned: it is much better to puncture the medial metatarsal vein.The caudal vena cava drains the liver, kidney, gonads, and oviduct. It forms ventral to the kidneys from the union of the common iliac veins that drain the pelvis and hindlimb (Figure 37-29/13). As described on page 805, some blood from the pelvis and hindlimb passes through the kidney (renal portal system) before reaching the caudal vena cava. Blood from the gastrointestinal tract reaches the liver by separate right and left hepatic portal veins that enter the respective lobes. The left vein drains the left and ventral parts of the stomach. The much larger right vein drains the right and dorsal parts of the stomach, the spleen, and the remainder of the tract through cranial and caudal mesenteric veins. The caudal mesenteric vein, connected to the caudal end of the renal portal system (Figure 37-29/22), also conveys a considerable amount of blood toward the kidney. Thus, some blood from the gastrointestinal tract may return to the heart without passing through the liver.