422 ANIMAL PHYSIOLOGY. 
ey 
more than tubules, coiling but 
Spe ‘| y \¢ slightly, ending by one extrem- 
I ity in a glomerulus and by the 
other opening into a long com- 
u mon efferent tube or duct. The 
————< __ glomerulus is, however, pecul- 
Vv iar to the vertebrate kidney. 
The graded complexity in ar- 
ry rangement, etc., of the tubes is 
aw 
In most fishes the kidney is 
a very elongated organ, though 
in the lowest it consists of little 
7B represented well in the figure 
below. It is a significant fact 
that the kidney of the human 
subject is lobulated in the em- 
bryo, which condition is persist- 
ent in some mammals (rumi- 
nants, etc.). 
As the lungs are the organs 
employed especially for the 
elimination of carbonic anhy- 
dride, so the kidneys are above 
Xx 
i 5 emer all others the excretors of the 
Fic. 327.Diagrammatic representation of . 
distribution of tubules cf kidney (after nitrogenous waste products of 
Huxley). C, cortical region ; B, bound- 7 ; 
ee ee ae 
are the main outflow tubules. . 
ing of secretion by the kidney 
it will be well to examine into the physical and chemical prop- 
erties of urine with some detail, especially on account of its 
great importance in the diagnosis of disease. 
>P 
URINE CONSIDERED PHYSICALLY AND CHEMICALLY. 
Urine is naturally a fluid of very variable composition, espe-. 
cially regarded quantitatively—a fact to be borne in mind in 
considering all statements of the constitution of this fiuid. 
Specific Gravity — Urine must needs be heavier than water, on 
account of the large variety of solids it contains. The average 
specific gravity of the urine for the twenty-four hours is 1015 
to 1020. It is lowest in the morning and varies greatly with 
the quantity and kind of food eaten, the activity of the lungs 
and especially of the skin, with emotions, etc. 
