SECRETION 



353 



chemical substance of the nature of a 

 hormone is produced by the action of the 

 carbohydrate on the mucous membrane 

 of the stomach, similar to the secretin of 

 the pancreatic mechanism. 



THE KIDNEY 



Owing to the peculiar arrangements 

 present, special description of the 

 mechanism of this organ is necessary. 



We have seen that its activity is 

 confined to the separation of substances 

 which already exist in the blood, with 

 the exception of hippuric acid, and even 

 in this case the chemical change merely 

 consists in the combination of glycine 

 with a benzoyl group, both supplied by 

 the blood. 



We have also discussed the function 

 of the glomeruli and come to the con- 

 clusion that the liquid leaving their 

 capsules is a filtrate from the blood, 

 having the same composition minus the 

 colloids. The urine as it leaves the 

 kidney, however, is much more concen- 

 trated and, as we have also seen, the 

 concentration does not affect all the con- 

 stituents equally. The problem now 

 before us is the way in which this change 

 is effected as the glomerular filtrate passes 

 along the tubules, which consist of a 

 series of tubes lined with cells of various 

 structure. 



To understand the evidence on the ques- 

 tion, a knowledge of the structure of the 

 kidney is necessary. This can be obtained 

 from Starling's book (1912, pp. 1264-1268) or 

 from the article by Metzner in Nagel's Hand- 

 buch (1907, 2) and it must be assumed in what 

 follows here. Fig. 94 will serve to give a 

 general idea of the arrangement of the tubules. 



In the higher animals the function 

 of the kidneys may be said to be of two 

 kinds. In the first place, non-volatile 

 products of metabolism, which are useless 

 or injurious, have to be removed. In the 

 second place, the osmotic pressure of the 

 blood has to be kept constant. This 

 osmotic pressure is due chiefly to the 

 salts, so that the excretion of salts must 

 be increased or diminished, according to 

 the amount taken in with the food, and 

 that of water adjusted in accordance 

 with that taken or lost in other ways. 

 In the lower animals, where the osmotic 

 pressure of the body fluids is that of the 

 solution in which they live, the first 

 function is the chief or only one, so that 

 we will consider this to begin with. 



23 



H 



FIG. 94. 



THE STRUCTURE OF THE KIDNEY. 

 (Huber.) 



M , Malpighian capsules containing glomeruli. 



V, Entrance of blood vessels into glomerulus. 



n, Neck. 



rf.c, Distal convoluted tubules, arising from malpighian 



corpuscles. 

 K, Spiral tubule. 

 d, Descending limb of Henle's loop. 



All the above are left white. 

 H, Bend of the loop, 

 a, Ascending limb of Henle's loop. 

 p.e, Proximal convoluted tubule. 

 j, Junctional tubule. 



These last lour parts are grey. 

 c, Collecting tubule. 

 B, Duct of Bellini, receiving a number of collecting 



tubules and opening into the cavity of the pelvis 



of the kidney. 



