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 

 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 OP 

 KIDNEY OF MAN. 



A, vertical section of the kidney ; 

 a, cortical substance ; b, 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. 



