EXCRETION OF MATTER FROM THE BODY 425 



dog generally causes a decreased flow of blood through the 

 kidney and practically stops the flow of urine, although the 

 tubules, as will be presently shown, still act. 



(c) In the frog the renal arteries supply the Malpighian 

 bodies, while portal veins from the posterior end of the animal 

 supply the convoluted tubules. Ligature of the renal arteries 

 stops the flow of urine, but the flow may be again induced when 

 urea is injected. 



(d) Even when this flow is induced, if dextrose or egg 

 albumin or peptone are injected into the blood, substances 

 which in the normal frog appear in the urine, they are not 

 excreted. 



These observations seem to show that the Malpighian bodies 

 have to do chiefly with the excretion of water and of certain 

 abnormal constituents of the blood, and that their activity 

 depends upon the rate of blood-flow through them. That it is 

 not due to a mere physical filtration under pressure is indicated 

 by experiments, in which the blood pressure in the kidneys of a 

 dog was raised by injecting large quantities of blood from 

 another dog, without the flow of urine being increased. That 

 it is a selective action of the epithelium seems to be proved 

 by the passage into the urine of such large molecules as those 

 of egg albumin and haemoglobin and of various pigments such 

 as carmine. 



The fact that, if a cannula connected with a manometer 

 be placed in the ureter, the secretion of urine stops when a 

 pressure of about 50 mm. Hg is reached, is not opposed to the 

 view that the formation of urine from the glomeruli is an active 

 secretion. In the salivary gland, where the formation of saliva 

 is undoubtedly due to the vital action of the cells, the same 

 result follows obstruction of the duct. 



The point of practical importance is that the secretion of 

 water takes place chiejly through the Malpighian bodies, and 

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

 arterial pressure, such as occurs in heart failure. The 

 decreased excretion of water may lead to the development 

 of dropsy. 



The Tubules. The alkaline urine formed in the Malpighian 

 bodies undergoes changes as it passes along the tubules. It 

 becomes acid and the various solids are increased, for urine 



