370 A MANUAL OF PHYSIOLOGY 



its molecular concentration cause the passage of water into it from 

 the blood* 



It ought to be remembered in this whole discussion that the 

 epithelium of ordinary glands derives its supplies of material from 

 the 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 membrane 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 glycerine and fatty acids, 

 but it has been a subject of discussion whether it all under- 

 goes this change or only a portion of it. The common view 

 has long been that the greater part of the fat escapes de- 

 composition, and, after emulsification by the soaps formed 

 from the liberated fatty acids, is absorbed as neutral fat by 

 the epithelial cells covering the villi. If an animal is killed 

 during digestion of a fatty meal, these cells are found to 

 contain globules of different sizes, which stain black with 

 osmic acid, are dissolved out by ether, leaving vacuoles in 

 the cell substance, and are therefore fat (Plate III., i). It 

 has always been difficult to explain how droplets of emulsi- 

 fied fat could get into the interior of the epithelial cells, and 

 yet it certainly passes into them, and not between them. 

 When fat is found in the cement substance between the 

 cells, it has been mechanically squeezed out of them by the 

 shrinking of the villi in preparation. This difficulty is 

 obviated if we suppose that the whole of the fat is split up 

 in the intestine, the products being absorbed in solution, the 

 glycerine as such, and the fatty acids either as soaps or in 

 the free state, or partly free and partly saponified. If this 

 is the true theory and the evidence of its truth has of late 

 years been continually growing neutral fat must again be 



* Although it is customary to speak of lymph as if it were perfectly 

 homogeneous, there is no experimental ground for supposing that the 

 tissue liquid in contact with the cells of different organs, or even the 

 tissue liquid in contact with one and the same cell at different parts of its 

 periphery, has a uniform composition. There are indeed certain genera. 1 

 considerations which show that this cannot be so. 



