MAMMALIA. 295 



Air consequently rushes into the larger air-passages from the 

 exterior. 



Expiration is the exact converse of this, but is largely passive, 

 the various muscles ceasing to contract. It is aided by the 

 elasticity of the lungs and walls of the thorax. As a result, the 

 air rushes out again. 



The air passes directly in and out of the larger air-passages 

 only. The rest is effected by diffusion, and the essential part of 

 respiration (exchange of gases) is carried on in the infundibula. 



Respiration is less active than in the pigeon, but here also the 

 body is maintained at a temperature (about 100F.) much above 

 that of the surrounding medium. 



7. The urino-genital organs consist of excretory and repro- 

 ductive organs. 



Excretory Organs (Fig. 89). In both sexes two oval, compact, 

 reddish-brown kidneys are closely applied to the dorsal wall of 

 the abdominal cavity, the right rather further forwards than the 

 left. The} 7 are covered ventrally by peritoneum. 



Each kidney presents a notch, the hilus, on its inner margin, 

 from which the ureter, a narrow tube with muscular walls, 

 proceeds. The ureters open obliquely into the dorsal side of 

 the pear-shaped urinary bladder, towards its narrow portion or 

 neck. The walls of the bladder are translucent and muscular. 

 It projects into the body-cavity just in front of the symphysis 

 pubis. 



A median horizontal section of either kidney shows that it is 

 differentiated into a marginal cortex, dark red in colour, with 

 numerous small dots (Malpighian bodies), and a central paler 

 medulla, which presents a striated appearance. The strise con- 

 verge internally towards a conical eminence, the urinary pyramid, 

 which projects into the pelvis, or dilated end of the ureter. 



The kidney is essentially made up of uriniferous tubules 

 (Fig. 88), each commencing with a Bowman's capsule, which with 

 its glomerulus constitutes a Malpighian body, and is situated in the 

 cortex. The neck of the capsule passes into a thickened glandular 

 part, also in the cortex, from which a narrow part runs into the 

 medulla, and then loops back into the cortex to dilate into a 

 second glandular part, which passes into a collecting part. The 

 collecting parts run towards the urinary pyramid, often uniting 

 in their course, and finally open upon it. 



