310 STRUCTURE OF THE KIDNEY. 
whilst the inner, 6, is termed medullary (or pith-like). In 
the cortical substance, no definite arrangement can be de- 
tected by the naked eye ; ‘it chiefly 
consists of a very intricate network 
a Of blood-vessels, surrounding the 
extremities of the tubes. But in 
ec the medullary substance we can 
trace a regular passage of minute 
¢ tubes, from the circumference to- 
wards the centre. They commence 
in the midst of the network of 
blood-vessels (B, a), and then pass 
down in clusters, nearly in a 
straight direction, and slightly con- 
verging towards each other, until 
each cluster terminates in a little 
B body, called the caly« or cup, which 
discharges the fluid it receives into 
the large cavity of the kidney, 
termed the pelvis or basin (4, ¢). 
From this it is conveyed away by 
the ureter d, which terminates in — 
Fig. 172.—Strucrure or raz the bladder. 
Kipney or May, 369. One of the most interesting 
A, vertical section of the kidney; circumstances in reference to the — 
@, cortical substance; 5, tubular . : : 
dubstance, a ealye and pelvis, Urinary secretion, is the very large © 
oa 
d, ureter, quantity of water which, in the © 
PP extremity of tie uriniferous higher animals, is got rid of through 
theirtermination in he calyx.” ‘RIS channel, and. the mean 
which it is drawn off. e eys 
seem to form a kind of regulating valve, by which the quan- ~ 
tity of water in the system is kept to its proper amount. The ~ 
exhalation from the Skin is liable to sustain great variations — 
in its amount from the temperature of the air around ; for 
when this is low, the exhalation is very much diminished 5 — 
and when it is high, the quantity of fluid that passes off im ~ 
this manner is increased (§ 371). Hence, if there were not 
some other means of adjusting the quantity of fluid in the 
blood-vessels, it would be liable to continual and very inju- 
rious variation. This important function is performed by the 
kidneys, which allow such a quantity of water to pass into 
