546 DR DAVY ON THE URINARY SECRETION OF FISHES. 
tion of fishes is very limited as to quantity; 2d/y, that it is commonly liquid ; 
3dly, that the nitrogenous compound eliminated is variable,—either urea or a 
lithate (the latter probably very seldom), or some nearly allied compound of 
azote. 
A brief glance at this secretion in other classes of animals may here not be 
out of place, as bearing on these conclusions. I need not dwell on the import- 
ance of the urinary secretion, denoted by its generality, and how, in all the great 
divisions of the animal kingdom in which it has hitherto been examined, viz., 
the mammalia, birds, reptiles, insects, spiders, the mollusca, it has been found to 
consist chiefly of compounds abounding in nitrogen, authorizing the commonly- 
received conclusion that the secerning organs are depurating in their function, 
and the main channel by which the excess of this element (nitrogen) is removed 
from the system. 
The differences however compatible with this intent, — differences in the 
nature of the secretion,—are not a little remarkable. [allude merely to the qua- 
lity—to the chemical ingredient; and they seem to be regulated more by the 
structure of the urinary apparatus, or secerning vessels, than by any other cir- 
cumstance, not even excepting the kind of diet, whether animal or vegetable, or 
an admixture of the two. 
In the mammalia, provided with an ample urinary bladder, the normal secre- 
tion is seen to be entirely liquid, and the principal ingredient, so far as it has yet 
been determined, always soluble urea: Such it has been found to be in man; such 
in the carnivorous animals; such in the herbivorous; with the addition, in that 
of some of them, of the hippuric acid. 
In birds, on the contrary, and in those reptiles which, like them, are desti- 
tute of a urinary bladder, viz., snakes and lizards, invariably the secretion, judg- 
ing from my own pretty extended experience, is chiefly solid,—a soft, plas- 
tic one, owing its consistence to admixture with water, and composed princi- 
pally of lithate of ammonia and lithic acid. Yet in others of the latter class, 
which have a receptacle corresponding to the urinary bladder, and destined to 
hold the secretion,* the secretion is fluid, as in the instance of the toads and 
frogs; and the nitrogenous matter eliminated is again the soluble urea. The 
same remark applies to the tortoises, with this difference, that sometimes, though 
their food be vegetable solid matter, flakes of a lithate are occasionally found 
suspended in the fluid contents of their urinary bladder. 
In insects, also in spiders and scorpions, all which, it is presumed, have no 
* Whether this receptacle be considered,—as it is by Mr T. R. Jones, in his General Outlines of 
the Animal Kingdom (p. 585)—the unobliterated remains of the allantois, or a true urinary bladder, 
its primary use, I apprehend, can hardly now be questioned, since all the later examinations that 
have been made of the fluid contained in it prove that in composition it is urinous, as stated above: 
whether, in the instance of the frog, it may not subserve to aid, as some distinguished physiologists 
suppose, by transpiration in keeping the skin duly moist, is open to question. 
