360 PHILOSOPHY OF ZOOLOGY. 
secretes a lubricating fluid, to protect the organ from the 
action of the contained fluid. The ureters enter the bladder 
towards its mouth on the dorsal side, passing through its 
coats in an indirect and tortuous manner. The bladder 
itself is retained in its place, partly by the folds of the pe- 
ritoneum, and partly by its own ligaments. These either 
arise from the neck of the bladder, and are attached to the 
pubis, or from its fundus, constituting the wrachus. ‘The 
urine, at stated intervals, is expelled from the bladder, 
through a canal termed the urethra, which accompanies 
the reproductive organs. 
The modifications of this system in the inferior classes of 
animals, are numerous. In none of them do the kidneys in 
their structure appear to consist of two parts. The ureters, 
while they pour their contents into a bladder of urine, in 
some reptiles and fishes, do not, in others, terminate in any 
common re ceptacle. In birds, in general, and many reptiles 
and fishes, the urine, before expulsion from the body, 1s mix- 
ed with the excrement, while in many fishes, it either passes 
out by a peculiar opening, or in a common passage with 
the melt or spawn. Nothing analogous to urinary organs 
has yet been detected in the mollusca or annulose animals, 
although in the dung of the caterpillars of several insects, 
traces of the peculiar principle of rine, or urea, have been 
detected. 
There isa singular compound body adhering to the upper 
part of the kidney in quadrupeds, termed the renal gland, 
whose use is pnknown. It is of a yellowish colour 
and firm consistence, and frequently contains a cavity filled 
with a dark serous fluid. In the foetus, it is equal in 
size to the kidneys, but in the adult, it 1s about one- 
fifth less. 
The wrine itself has been repeatedly examined by diffe- 
rent chemists, and a variety of products obtained from its 
