THE PIGEON. 235 



portion of the sciatic artery, and debouches into the renal 

 portal vein. 



226. The posterior (or inferior) mesenteric artery 

 (p.m.a\ springing from the dorsal aorta at about the level 

 of the posterior border of the kidneys, and going to supply 

 the rectum and cloaca : beyond its origin the aorta becomes 

 the caudal artery (c), which passes backward parallel and 

 ventral to the caudal vein, and from the same point the 

 internal iliac arteries (/.//), are given off, which 

 accompany the corresponding veins ( 218). 



227. The renal arteries, of which there are three to each kidney : 

 an anterior (r.a.i), springing from the dorsal aorta just anterior to the 

 origin of the femoral artery, and supplying the anterior lobe ; a middle 

 (r.a.2), arising from the sciatic artery near the point where it enters the 

 kidney and passing forwards to the middle lobe ; and a posterior (r.a.3), 

 arising from the sciatic a little external to the last, and passing backwards 

 to the posterior lobe. 



228. The origin of the coeliac (cos. a) and anterior 

 mesenteric arteries (a.m.a) ( 200, 203), from the 

 dorsal aorta. 



XXXV. Remove the ventral wall of the cloaca, and 

 note : 



229. Its division into three compartments (Figs. 52, 53, 

 and 55), separated from one another by incomplete parti- 

 tions ; the anterior chamber (d.\) is almost globular when 

 distended, and receives the rectum ; the middle chamber 

 (cl.2) is small, separated from the first by an annular fold 

 of the cloacal wall, and receives the urinary and genital 

 ducts ( 230 232); the posterior chamber (d.$) is 

 separated from the middle by a low annular ridge, contains 

 the aperture of the bursa Fabricii ( 233), and opens 

 externally by the vent. 



230. The small round apertures of the ureters (ur) 

 in the dorsal wall of the middle chamber. 



