264 DISEASE IN WILD MAMMALS AND BIRDS 
of two or three large lobes lying in concavities of the 
sacrum, each lobe being made up of tiny lobules. The 
latter appear to the unaided eye as fairly distinct 
divisions whether viewed on the exterior or by cross 
section. There is a cortex and a medulla to each, the 
separation being clear in a large specimen, vague in a 
small one. These lobulations are quite well observed 
when the kidney is full of urates, a common finding in 
birds. Magnification of the a\aan kidney reveals an 
apparently simpler tubular arrangement than is found 
for mammals (2) (3), yet the relation of vascular plexuses 
and secreting tubules remains similar. 
From the standpoint of comparative pathology atten- 
tion can be drawn to the glomerulus, to the interstitial tis- 
sue, and to the character of the epithelium. The tuft of 
intricately wound cajjillaries called the glomerulus has 
always been ^aewed as the part of the secretory unit 
cliiefly concerned in urine production whether one accept 
the older idea that it excretes only fluid or the modern 
belief of many observers that all parts of the urine go 
out through it. In the mammal the tuft is closely wound, 
is surrounded by a distinct space and a limiting membrane 
of appreciable -width called Bo^\^nan's capsule; all this 
so-called Malpigliian body has a breadth varying from 
120 to 300 micra. There is however great variation in 
the size of this body when seen in the peripheral and 
deeper zones of the same organ, amounting at times to 
seventy per cent, of the diameter. The capillary con- 
geries forming the avian tuft is by no means so delicate 
and one can see individual capillaries with more ease. It 
may be impossible to discover a space between the tuft 
and its exceedingly delicate capsule, the latter being 
usually applied closely to the vascular corpuscle. The 
whole breadth varies from 70 to 140 micra mth an average 
(2) Policard, C. R. Assoc. Anat., 1910, 12, 57. 
(3) Huber, Anat. Record, 1916, 10, 201. 
