150 THE VASCULAR SYSTEM 



pressure does not per se produce oedema in the healthy tissues. 

 To show to what an extreme limit this view may be carried, the 

 following example may be cited. Asher, a chief opponent of the 

 mechanical theory of lymph formation, has sought to overthrow 

 it by a number of ingenious experiments, of which the following is 

 one. He injected into the vein of a dog a concentrated solution 

 of sugar, and immediately killed the animal. The lymph flow from 

 the thoracic duct was measured, and this, in spite of the animal 

 being dead, increased from 4 to 37 c.c. per 10 minutes. In this 

 case, says Asher, there can be no question of a filtration pressure 

 because the animal is dead and the circulation at an end. In an 

 argument upholding the filtration hypothesis, Bainbridge suggests, 

 however, that the sugar, by raising the osmotic pressure of the 

 blood, drew water from the tissues into the blood, and thereby 

 increased the volume and the mean (residual) hydrostatic pressure 

 in the vascular system. " The capillary pressure after death will 

 therefore," he says, " be unusually high, and there will be an 

 excessive transudation of lymph, so that on analysis, the 

 experiment is seen to support, rather than to oppose, Starling's 

 views." 



Now to make the mechanical theory possible we must suppose 

 that the blood is driven through a system of rigid tubes with a 

 sieve-like structure. If water be driven under pressure through a 

 coil of hose which leaks, and the coil of hose be sunk in a tub of 

 water, then it is. true water will filter through and the tub will 

 overflow. But in an organ of the body the conditions are quite 

 otherwise, and an effective filtration pressure cannot exist because, 

 as the writer has proved in the case of the brain, the blood in the 

 capillaries, the tissue cells and lymph are at one and the same 

 pressure, viz. the capillary-venous pressure. 



Let us take the example of the salivary gland. Asher has 

 pointed out that while lymph flows in increased amount when the 

 gland is thrown into secretory activity, from an atropinised gland 

 no lymph flows when the chorda tympani is excited, although the 

 arteries dilate, and the capillary pressure is raised. Here is an 

 experiment, he says, which is against the filtration hypothesis, 

 for capillary pressure is raised and yet lymph does not flow. The 

 experiment may prove Asher's chief contention, that it is functional 

 activity with the consequent production of metabolites of high 

 osmotic pressure which determines the flow of lymph, but is not 



