462 FORMATION OF LYMPH 



Heidenhain, who advanced the theory that lymph is secreted by 

 the endothelium of the blood capillaries. One of Heidenhain's 

 strongest arguments in favour of his secretion theory was the 

 existence of substances which, when injected into the blood, in- 

 creased the flow of lymph from the thoracic duct of the dog without 

 affecting appreciably the arterial pressure. He divided these so- 

 called lymphagogues into two classes: (i) Substances like peptone, 

 extracts of the head and liver of the leech, extract of crayfish 

 muscle, egg-albumin, etc., which cause not only an increase in the 

 rate of flow, but an increase in the specific gravity and total solids 

 of the lymph; (2) crystalloid substances, like sugar, salt, etc., which 

 cause an increased flow of lymph more watery than normal. 



Starling has shown that, although the lymphagogues of the second 

 class do not raise the arterial pressure, they do, by attracting water 

 from the tissues arid thus causing hydraemic plethora (an excess of 

 blood of low specific gravity), bring about a marked rise of venous, 

 and therefore, what is the important thing for lymph filtration, of 

 capillary pressure. But it can be demonstrated that vaso-dilata- 

 tion with increase of capillary pressure is not in itself sufficient to 

 increase the formation of lymph. We have seen, e.g. (p. 177), that 

 when the chorda tympani nerve is stimulated in the dog the arteri- 

 oles of the submaxillary gland are dilated, and no doubt the pres- 

 sure in the capillaries is increased. No increased flow of lymph, 

 however, takes place from the submaxillary lymphatics during even 

 prolonged excitation of the chorda, nor do the lymph spaces of the 

 gland become distended (Heidenhain). In the horse also the spon- 

 taneous flow of lymph from the quiescent parotid is not appreciably 

 altered by excitation of the secretory nerves of the gland or by 

 pilocarpine (Carlson). There is every reason to believe that during 

 active secretion of saliva tissue liquid is really formed from the 

 blood in increased amount, and that it is from the tissue spaces 

 that the gland-cells directly obtain the increased supply of water 

 and other substances necessary to sustain the increased secretion. 

 But a balance is maintained between the production of tissue liquid 

 and its removal by the gland-cells. When the gland is quiescent, 

 the small amount of tissue liquid normally formed from the blood 

 capillaries for the nutrition of the cells is balanced by, upon the 

 whole, an equal amount of lymph secreted from the tissue spaces 

 into the lymph capillaries. 



We may say, indeed, that the closed lymphatic system has for 

 its great function the regulation of the quantity and quality of the 

 tissue liquid. In glands with an external secretion increased irriga- 

 tion of the tissue spaces from the blood does not as a rule lead to 

 increased flow of lymph, because the surplus fluid is required to 

 form the secretion. In other organs, however, such as the muscles 

 and the ductless glands, it is probable that the augmented irriga- 



