EXCRETION 40* 



urine which follows the absorption of a large quantity of 

 water is due to a mere process of filtration than there is to 

 believe that filtration, and not selective secretion, is the 

 cause of the gush of saliva which precedes vomiting, or the 

 sudden outburst of sweat on sudden and severe exertion. 

 It is true that the direct introduction of water into the 

 blood, or its attraction from the lymph spaces when the 

 osmotic pressure of the blood is increased by the injection 

 of substances like urea, sugar and sodium chloride, may 

 cause a condition of hydr&mic plethora, and that this plethora 

 may sometimes be associated with an increase of pressure in 

 the capillaries in general, and therefore in the vessels of the 

 Malpighian tuft. It may also be admitted that such an 

 increase of pressure might be accompanied by an increased 

 filtration of water and salts into the Bowman's capsule. 

 But who will believe that the addition of a tumbler of water, 

 absorbed from the alimentary canal, to 5^ litres of blood 

 circulating in a system of vessels whose capacity can and 

 does vary within wide limits, should cause in the capillaries 

 of the kidney an increase of pressure exactly proportional to 

 the increase in the elimination of water in the urine, lasting 

 for the same time and disappearing at the moment when 

 the normal composition of the blood is restored ? Nor is 

 it easier to explain on any filtration hypothesis how it is 

 that in a starving animal, the quantity of inorganic sub- 

 stances eliminated in the urine drops almost to zero, while 

 the proportional amount in the blood and tissues is little, 

 if at all, affected. Such facts suggest that the secreting 

 cells of the kidney are stimulated by the contact of blood 

 or lymph in which the normal constituents are present in 

 too small or in too great amount, and that the strength 

 of the stimulus is proportional to the degree of deficiency 

 or excess. 



As to the nature of the mechanism thus set in motion, and 

 the series of events that take place as the constituents of 

 the urine journey from the interior of the bloodvessels to the 

 lumen of the tubules, we know no more than in the case of 

 other glands. This alone is clear, that the separation of the 

 urine from the blood implies the performance of a large 



