Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/99

Rh Remove both kidneys from the body. With a sharp razor or scalpel divide one kidney into its dorsal and ventral halves, and bisect the other by a transverse cut passing through the hilum. The cut surfaces reveal the outer cortical and inner medullary regions of the kidney. Within the latter note the dorsoventrally flattened papilla which projects into the internal cavity, or sinus, of the kidney. The urine collects in the sinus and drains into the ureter through its funnel-shaped anterior end, the pelvis, which is located at the hilum of the kidney. Insert a fine bristle into the pelvis where it surrounds the tip of the papilla and by careful probing locate the lumen of the ureter.

Exercise XXII. Draw the cut surfaces of the bisected kidneys.

The spermatozoa, or male sex cells, are produced in the two testes, which develop in the embryo near the attachment of the mesentery to the dorsal side of the abdominal cavity. In the lower vertebrates the testes remain through- out life near their embryonic position, but in most mammals they come to lie in an integumentary sac, the scrotum, ventral to the anus. This scrotal sac incloses two extensions of the coelom, each of which communicates in the embryo with the abdominal cavity through the inguinal canal. This canal marks the position of the scrotal evagination. The testes usually descend from the abdominal cavity, through the inguinal canals, into the scrotal sac, though in the whale and elephant they remain in the abdominal cavity. Such a condition (cryptorchism) occasionally occurs in man. The inguinal canal remains open in marsupials, bats, rodents (including the rat), insectivores, etc. The descent of the testes in these animals is temporary, and at the close of the breeding season the testes are withdrawn into the abdomen by the cremaster