EXCRETORY SYSTEM 809 



the abdominal cavity by a partly muscular diaphragm, 

 which is supplied by two phrenic nerves, arising from the 

 fourth cervical spinal nerves. By its contraction the 

 diaphragm alters the size of the thoracic cavity, and thus 

 shares in the mechanism of respiration. At the top of 

 the trachea lies the complex larynx, the seat of the voice 

 in Mammals. 



Anteriorly the larynx is supported on its sides and beneath by the 

 thyroid cartilage ; behind this lies the ring-like cricoid ; dorsally to the 

 cricoid are two small triangular arytenoids. 



Within the larynx there are stretched membranous bands — the vocal 

 chords. Beside the larynx is the paired thyroid gland. 



Excretory system. — This includes the blood-filtering 

 kidneys, their ducts the ureters, and a reservoir or bladder, 

 into which these open. The kidneys and their ducts are 

 formed from the metanephros and metanephric ducts of 

 the embryo. The bladder arises as a diverticulum from 

 the hind end of the gut, being in fact a remnant of the 

 intra-embryonic part of the allantois. It loses its con- 

 nection with the gut, and the ureters which originally 

 opened into the rectum follow the bladder and open 

 into it. 



The kidneys are dark red ovoid bodies lying on the 

 dorsal wall of the abdomen ; the one on the left is farther 

 down than that on the right, because of the position of 

 the stomach on the left side. When a kidney is dissected, 

 a marked diflFerence is seen between the superficial cortical 

 part and the deeper medullary substance. On papillae or 

 pyramids in the very centre the coiled excretory tubules 

 open, and empty the water and w^aste products into the 

 " pelvis " or mouth of the ureter. 



The ureters run backward along the dorsal wall of the 

 abdomen, and open into the bladder, a thin-walled sac 

 lying in front of the pelvic girdle. 



In front of each kidney lies a yellow suprarenal body. 



Reproductive organs. — (a) Male. — The testes arise on 

 the dorsal abdominal wall near the kidney, but as the 

 rabbit becomes sexually mature, they are loosened from 

 their original attachment, and pass out on the ventral 

 surface, as if by a normal rupture, into the scrotal sac. 

 A spermatic cord, consisting of an artery, a vein, and a 



