412 A MANUAL OF PHYSIOLOGY 



plained, and which at present, for the want of a more precise term, 

 we must attribute to secretory activity. 



Although no definite lymph-secretory nerve-fibres have as 

 yet been discovered, it is possible that they exist (Sihler). As 

 already pointed out, the same volume of liquid as escapes into 

 the ducts of the active submaxillary gland must, upon the whole, 

 pass out of the blood capillaries. On what principle shall we 

 distinguish one only of these processes as physiological secretion ? 

 They begin together when the chorda tympani is stimulated. 

 A drug which paralyzes secretory nerve-endings abolishes both 

 effects. The simplest explanation is that the chorda contains 

 secretory fibres which influence the formation both of saliva and 

 of the tissue liquid from which it is recruited ; and, so far as this 

 consideration goes, it is just as logical to consider the increase 

 in the supply of tissue liquid as the cause of the increase in the 

 flow of saliva as to consider the increased salivary secretion as 

 the cause of the increased flow of liquid into the tissue spaces. 

 The increased flow of liquid may be brought about either by an 

 action of the nerve on the gland-cells, causing them to produce 

 a hormone, which then affects the blood capillaries (Carlson) , or 

 by a direct action on the capillary endothelium. The advantage 

 to cells engaged in the active secretion of saliva of being immersed 

 in an abundant bath of tissue liquid is obvious. The post- 

 mortem flow of lymph, which may continue in some cases long 

 after complete cessation of the circulation for an hour after 

 injection of dextrose to produce hydraemic plethora ; for as 

 much as four hours after injection of extract of the strawberry, 

 which is a lymphagogue of Heidenhain's first group (Mendel 

 and Hooker) is a phenomenon whose relation to normal lymph 

 formation has not been definitely settled. 



It ought to be remembered in this whole discussion that the 

 epithelium of ordinary glands derives its supplies of material 

 from the tissue lymph. The vicissitudes of blood-pressure affect 

 it only in a secondary and indirect manner. On the other hand, 

 the endothelial cells of the capillaries are in direct contact with 

 the blood. And it is interesting to observe that in this respect 

 the glomeruli of the kidney and the alveoli of the lungs (if the 

 endothelial lining of Bowman's capsule and the alveolar mem- 

 brane are assumed to be complete) take a middle place between 

 the glands proper and the quasi-glandular capillaries. 



Absorption of Fat. It has been already mentioned that fat 

 is split up in the intestine into glycerin and fatty acids, but it 

 has been a subject of discussion whether it all undergoes this 

 change or only a portion of it. The common view has long been 

 that the greater part of the fat escapes decomposition, and, 

 after emulsification by the soaps formed from the liberated fatty 



