FORMATION OF URINE 575 



pressure, due to the colloids, is decreased formation of urine goes 

 on at a lower pressure than 50 mm. Hg. He showed that the 

 formation of urine is increased by injecting hypertonic solutions 

 of glucose which caused a hydraemic plethora by producing an 

 endosmosis from the tissues to the blood (p. 450), and he further 

 proved that this is not simply the result of an increased pressure 

 due to the increased volume of the blood by demonstrating that 

 the injection of fluids of high colloidal content did not increase 

 the flow of urine. 



It is thus clear that filtration is the main factor in the 

 formation of urine in the Malpighian bodies, and the membrane 

 of cells through which this occurs must form a semi-permeable 

 membrane which prevents the passage of the colloidal proteins 

 of the blood. 



On the other hand, that a selective action of the epithelium 

 is involved seems to be suggested by the passage into the urine 

 in Bowman's capsule of such large molecules as those of egg 

 albumin and haemoglobin and of various pigments such as 

 carmine. 



The point of practical importance is that the secretion of 

 water takes place chiefly through the Malpighian bodies, and 

 that this is reduced or stopped by a fall in the general 

 arterial pressure, such as occurs in failure of the heart. The 

 decreased excretion of water may lead to the development of 

 dropsy. 



B. The Tubules. 



In the filtrate, the percentage of the various substances in 

 solution cannot be higher than it is in the blood plasma. 



But, as already stated, the concentration of most of the con- 

 stituents of the plasma is generally enormously increased in 

 the urine while the reaction is acid. 



These changes must be effected in the tubules. 



There are manifestly two ways in which they might be 

 brought about. 



1. By the secretion of solids by the epithelium from the 

 blood to the urine (Bowman and Heidenhain). 



2. By the absorption of some of the water filtered through 

 the Glomeruli back into the blood (Ludwig). Since the degree 



