310 
STRUCTURE OF THE KIDNEY. 
whilst the inner, 6, is termed medullary (or pith-like). In 
the cortical substance, no definite arrangement can he de- 
A tected by the naked eye ; it chiefly 
consists of a very intricate network 
of blood-vessels, surrounding the 
extremities of the tubes. But in 
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 
body, called the calyx or cup, which 
discharges the fluid it receives into 
the large cavity of the kidney, 
termed the pelvis or basin (a, c ). 
From this it is conveyed away by 
the ureter d, which terminates in 
the bladder. 
369. One of the most interesting 
circumstances in reference to the 
Urinary secretion, is the very large 
quantity of water which, in the 
higher animals, is got rid of through 
this channel, and the means by 
which it is drawn off. The kidneys 
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 ; 
and when it is high, the quantity of fluid that passes off in 
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 
Fig. 172.— Structure of the 
Kidney of Man. 
A, vertical section of the kidney; 
a, cortical substance; &, tubular 
substance; c, calyx and pelvis; 
d, ureter. 
B, portion of the gland enlarged; 
a, extremity of the uriniferous 
tubes; b, straight portion; c, 
their termination in the calyx. 
