SECTION VIII 
THE URINARY TRACT 
The Kidney. 
The kidneys, ureters, bladder and urethra remain com- 
parable in all mammals excepting the monotremes where 
there is no urinary passage through genital openings, 
the urine being ejected through the vesicoanal pouch, a 
sort of cloaca. In the bird the first two parts remain as 
in mammals while the ureters terminate in a hernia-like 
pouch of the rear wall of the cloaca. It would seem 
from this arrangement that ureteral transmission of 
infection from the anal area to the kidneys would be 
facilitated in the lowest mammals and in the Aves. Varia- 
tions in size, shape and position exist to a minor extent in 
the higher orders but in all forms, the system remains a 
post-peritoneal structure. 
Differences in construction are to be seen for example, 
in the single pyramidal kidneys of marsupials and certain 
rodents, in the lobulated organ of bears, cattle and seals, in 
the twisted viscus of horses, but these gross appearances 
do not destroy the uniform scheme upon which the func- 
tionating unit is built. The single-lobed kidney discharges 
all its collecting tubules into one calyx while the multiple 
pyramids of the lobulated organ are fitted with individual 
calices which in turn empty into the pelvis proper; this 
is true whether the lobulations are retained, as in the 
bear, or are smoothed out in the course of development as 
in man. The secretory tubule remains in essentially the 
same form in all kidneys ; the modern idea of its anatomy 
may be found in the work of Iluber(l). 
The most decided example of the lobulated kidney is 
to be found in the class Aves, wherein the organ consists 
(1) Anatomical Record, 1917, 13, p. 305, On the morphology of the 
renal tubule in the vertebrates. 
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