EXCRETION BY THE KIDNEY. 419 
The functional activity of the skin varies with the tempera- 
ture, moisture, etc., of the air and certain internal conditions; 
especially is it important to remember that it is one of a series 
of excretory organs which act in harmony to eliminate the 
waste of the body, sothat when one functions more the other 
may and usually does function less, 
The protective function of the skin and its modified epithe- 
lium (hair, horns, nails, feathers, etc.) is in man slight, but very 
important in many other vertebrates, among which provision 
against undue loss of temperature is one of the most constant- 
ly operative, and enables a vast number of groups of animals 
to adapt successfully to their varying surroundings. 
EXCRETION BY THE KIDNEY. 
The kidney in man and other mammals may be described as 
a very complex arrangement of tubes lined with many different 
forms of secreting cells, sur- 
rounded by a great mesh- 
work of capillaries, bound 
together by connective tis- 
sue, the quantity varying 
with the animal, and the 
whole inclosed in a capsule. 
The organ is well supplied 
with lymphatics and nerves. 
Though the tubes are so 
complex, the kidney may be 
divided into zones which 
contain mostly but one kind 
of tubule. 
Comparative — Among the 
lowest forms, the Infusori- 
ans and Celenterates, ex- 
cretory organs have not 
been definitely traced. In 
the Vermes, organs known 
as nephridia (segmental or- 
gans, see Figs. 253, 257) are ; ; ; 
Fig. 324.—Vertical section of kidney (after Sap- 
supposed to act the part of pes). 11,2233, 3, 4, 4, 4, 4, pyramids of 
a E a alpighi : 5, 5, 5,5, 5, 5, apices of pyramids, 
the kidney in some fashion. surrounded by calices ; &:6: columns of Ber. 
: tin ; 7, pelvis of kidney ; 8, u Tr extremity of 
These are long, often coiled _ ureter.” ONE, es 
