368 



A MA NUA L OF PHYSIOL G Y 



pass, by physical processes alone, from serous cavities into the blood, 

 and from the blood into serous cavities, it is still necessary to inquire 

 whether anything else is concerned in the formation of the lymph ; 

 all the more as the recent researches of Asher throw grave doubt 

 on the common view that these sacs are merely expanded lymph 

 spaces, and indicate that the liquid found in them has a different 

 origin from lymph. 



Formation of Lymph. The teaching of Ludwig, that lymph is 

 formed by the filtration and diffusion of the constituents of blood- 

 plasma through the walls of the capillaries into the tissue spaces, was 

 first seriously called in question by Heidenhain, who advanced the 

 theory that it is secreted by the capillary endothelium. 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 without 

 affecting appreciably the arterial 

 pressure. He divided these so-called 

 lymphagogues into two classes: (i) 

 substances like peptone, leech-ex- 

 tract, extract of crayfish, egg-albu- 

 min, 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) crystal- 

 loid 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 



u-. inyti ui T..UI u i ii uru L, MI iii'-ii u in wj>t;i- , T , ,1 i 



ing the viiius the outer edge of the classdo not raise the arterial pressure. 

 cells is striated ; b, central lacteal of they do, by attracting water from the 



villus ; e. unstriped muscular fibres ; d. tissues and thus caus i ng hydrsemic 

 nmcin-formmg goblet-cell. 



plethora (an excess of blood of lo\v 



specific gravity), bring about a marked rise of venous, and there- 

 fore, what is the important thing for lymph filtration, of capillary 

 pressure. The action of the first class of lymphagogues, which 

 cannot be explained in this way because the pressure in the capillaries 

 is not increased, he attributes to an injurious effect on the capillary 

 endothelium (and especially on the endothelium of the capillaries of 

 the liver, since nearly the whole of the increased lymph-flow comes 

 from that organ), which increases its permeability. Starling's expla- 

 nation is supported by various facts, but ir. is not easy to distinguish 

 an increase of permeability produced by lymphagogues from an 

 increase of secretive activity of the endothelial cells. Hamburger, 

 too, has brought forward results which it is difficult to reconcile with 

 a theory of filtration even for the second class of lymphagogues. 

 Further. Heidenhain has shown that some time after injection of a 

 crystalloid substance, like sugar, into the blood, a greater percentage 



FIG. ii2. VERTICAL SECTION OF A 



V ILL us (CAT) x 300. 

 a, layer of columnar epithelium cover- 



