408 



A MANUAL OF PHYSIOLOGY 



(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 attract- 

 ing water from the tissues and 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 nitration, of capillary pressure. But it can be demon- 

 strated that vaso-dilatation with increase of capillary pressure 

 is not in itself sufficient to increase the formation of lymph. 

 We have seen, e.g. (p. 163), that when the chorda tympani nerve 

 is stimulated in the dog the arterioles of the submaxillary gland 



are dilated, and no doubt the 

 pressure in the capillaries is in- 

 creased. No increased flow of 

 lymph , however, takes place from 

 the submaxillary lymphatics dur- 

 ing even prolonged excitation of 

 the chorda, nor do the lymph 

 spaces of the gland become dis- 

 tended (Heidenhain) . In the horse 

 also the spontaneous 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 in- 

 creased amount, and that it is 

 from the tissue spaces that the 

 gland-cells directly obtain the in- 

 creased 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 irrigation of the tissue spaces from the blood does not, 

 as a rule, lead to increased flow of lymph, because the surplus 



FIG. 155. VERTICAL SECTION OF A 



VILLUS : CAT. x 300. 

 a, layer of columnar epithelium 

 covering the villus the outer edge 

 of the cells is striated ; 6, central 

 lacteal of villus ; c, unstriped mus- 

 cular fibres ; d, mucin - forming 

 goblet -cell. 



