274 A MANUAL OF VETERINARY PHYSIOLOGY 



of it at once cause a flow of lymph. The only part of the body 

 which produces a continuous flow of lymph during rest is the 

 alimentary canal. Though no lymph is yielded by a resting 

 limb, yet the chemical changes in the tissue are still occurring, 

 oxygen is being absorbed, carbonic acid and other waste products 

 got rid of, but their channel of excretion is effected by the blood- 

 vessels. 



The Secretory Theory of lymph formation is based on a know- 

 ledge of the secretory activity of epithelium in general. It was 

 natural to regard the endothelial lining of the capillary vessels as 

 the possible seat of secretory activity, as was known to be the 

 case in other tissues ; and when Heidenhain was able to show 

 that by the injection of certain substances into the blood he was 

 able to increase the flow of lymph without increasing the arterial 

 pressure, it appeared that the solution of the vexed question of 

 lymph formation was at hand. 



Heidenhain found that the injection into the blood of peptone, 

 extracts of leech, crayfish, muscle, egg-albumin, etc., increased 

 the rate of lymph flow, and also the total solids in the lymph 

 obtained from the thoracic duct. He called these substances 

 lymphagogues, and divided them into two classes, the first class 

 being the above, while the second consisted mainly of crystalloid 

 bodies such as sugar and salt, which, though increasing the total 

 bulk of lymph produced, rendered it more watery than usual. 



Regarding Class i, Heidenhain believed they contained a 

 specific substance which stimulated secretion. Starling showed 

 that the increased secretion was derived from the liver ; he 

 believed the extracts acted pathologically on the walls of the blood 

 capillaries of the gland, and rendered them more permeable ; 

 hence the increased flow of lymph. The second class of lympha- 

 gogues was believed by Heidenhain to act by attracting water 

 from the tissues, and hence increasing the bulk of lymph. Star- 

 ling, however, maintains in this case that they act by increasing 

 the osmotic pressure of the circulation, so that water is attracted 

 from lymph and tissues into the blood by osmosis. The excess 

 of fluid thus produced in the blood causes a rise in the capillary 

 pressure, especially that of the abdominal area, followed by in- 

 creased transudation from the capillaries into the lymph- vessels. 



At present it is not possible to decide between the rival theories 

 of lymph formation ; it may be proved that under given condi- 

 tions both play a part in the process. It seems impossible to 

 exclude the living activity of the cell-body, so strongly urged in 

 the matter of other secretions, while it is equally certain that there 

 are other conditions which are only possible of explanation on 

 a physical basis. 



The Movement of Lymph is largely brought about by muscular 



