310 FUNCTIONS OF NUTRITION. 



the parts occupied by them a well-marked opaque appearance ; and they 

 thus form, in a thin section of the gland, elongated, opaque tracts, 

 separated by transparent interspaces, and communicating with each 

 other at frequent intervals. These tracts are called the medullary cords 

 of the lymphatic gland. They are the only vascular parts of the organ ; 

 as the capillary blood-vessels never pass beyond them into the inter- 

 vening transparent spaces. The transparent spaces are the lymph- 

 paths, or the channels by which the lymph traverses the gland from 

 its afferent to its efferent vessels. The afferent lymphatic vessels, 

 according to the testimony of nearly all observers, after ramifying 

 upon the outer surface of the gland, penetrate its fibrous envelope 

 and become continuous with the transparent portions of its substance. 

 This is shown by injections of the gland from the afferent vessels ; 

 and Kdlliker has demonstrated a connection of the same channels 

 with the efferent vessels, by injecting them from the substance of the 

 gland. 



The cause of the transparent appearance presented by the lymph- 

 paths in thin sections of the gland is that their lymph-cells are easily 

 detached by manipulation, while those of the medullary cords are more 

 firmly fixed in the fibrous mesh-work and do not so readily yield to a 

 displacing force. It has been found by Kolliker that a watery or 

 serous fluid, injected through the substance of the gland under mod- 

 erate pressure, will also displace these cells and leave the parts 

 which they occupied nearly clear. It is for this reason that the 

 lighter spaces in the lymphatic glands are regarded as the chan- 

 nels by which the lymph passes from the afferent to the efferent 

 vessels, the lymph-cells being detached by this current from their 

 place of growth and carried onward through the lymphatic system. 



Transudation and Absorption by Animal Tissues, 

 If a fresh animal membrane be securely fastened over the lower 

 end of a glass tube, the tube filled with a solution of various sub- 

 stances, and immersed in an exterior vessel of pure water, so that the 

 membrane is a diaphragm, with the water on one side and the solution 

 on the other, it is found that different substances penetrate the mem- 

 brane and pass through it to the water with different degrees of rapidity. 

 As a rule crystallizable substances, such as mineral salts, glucose, or 

 urea, pass with facility ; while non-crystallizable matters, such as albu- 

 men, starch, or gum, pass either not at all, or with difficulty. The 

 former are called "diffusible" substances, because they pass through 

 the membrane and become diffused in the water beyond ; the latter 

 are "non-diffusible," and do not appear in the exterior liquid, which 

 consequently maintains its purity. This distinction is not absolute, 

 since nearly all soluble substances may be made to transude in some 

 degree by increasing the pressure on the corresponding side of the 

 membrane ; but the difference in this respect is often very great. 



