170 THE VASCULAR SYSTEM 



blood beforehand so as to compensate for the incoming of the 

 tissue fluid, no noteworthy increase in the lymph flow occurred. 

 Objections to the filtration hypothesis have already been fully 

 stated ; it remains to suggest another explanation for Starling's 

 results. The following hypothesis, for the present, may be found 

 adequate. 



The injection of the sugar increases the osmotic energy of the 

 blood above that of the tissues, and draws water into the vascular 

 system. The capillaries are better filled and the surface energy 

 of the tissuo spaces is increased in proportion as the surface tension 

 is increased in the capillaries. Sugar passes through the permeable 

 film formed by the capillary wall into the tissue spaces and cells, 

 and thus the osmotic pressure of the blood falls. Surface energy 

 then gets the upper hand and water passes from the distended 

 capillaries into the tissue spaces. This occurs particularly in the 

 highly vascularised abdominal organs where the flow of lymph is 

 furthered by the respiratory pump. 



HYPER^MIA 



The blood supply of each organ is in relation to its functional 

 activity, and thus while half of the whole blood quantum is esti- 

 mated to be in the viscera, and the blood they contain may be 

 equal to one-fifth of their weight, that within the locomotor organs 

 in the resting state of the animal is said to be not more than 

 2 to 3 per cent, of their weight. The capillaries of the skin are 

 normally far from filled. The effect of an irritant on the vessels 

 of the skin or conjunctivas and on the mucous membrane of the 

 lip demonstrates the striking difference in the capillary supply. 

 Hypersemia may be active and arterial in origin, or passive and 

 due to venous congestion. Active hyperaemia may be caused by 

 increased resistance in other vascular fields the arterial supply 

 to the brain is increased by constriction of the splanchnic area 

 or it may result from vaso-dilatation, and this in its turn may be 

 produced by the vase-motor nerves, or by physical or chemical 

 agents acting locally, e.g. effect of heat and cupping on the skin, 

 pilocarpine on the salivary glands and diuretics on the kidneys. 

 It is suggested that the products of secretion cause vaso-dilatation 

 in the salivary gland when the chorda is excited, just as excretory 

 products dilate the renal vessel. 



