CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 493 



alveoli, communicating laterally with each other to a small extent- 

 only, but which in the medulla rapidly diminish in size and, 

 opening freely into each other on all sides, form a labyrinth. At 

 the hilus the medulla comes to the surface of the gland, but 

 elsewhere is separated from the surface by the cortex. The 

 number of and regularity of division among the alveoli, and the 

 sharpness of distinction between the cortex and the medulla 

 differ in the glands of different animals. 



Each alveolus of the cortex consists in its central part, 

 constituting about two-thirds or more of the whole chamber, of a 

 mass of adenoid tissue crowded with leucocytes ; this mass which 

 follows the form of the chamber, is wholly like, in fact repeats 

 almost exactly the structure of the mass of adenoid tissue of a 

 solitary follicle of the intestine ; it is spoken of as the follicular or 

 glandular substance or more briefly the follicle, of the alveolus. 

 This follicle is separated on all sides from the capsule and trabeculse 

 which form the walls of the alveolus (or from the trabeculse alone 

 where as in some cases the alveolus is a small one lying between 

 the larger superficial alveoli and the true medulla) by a space 

 which is occupied as a rule not by true adenoid tissue but by a 

 coarser more open reticular tissue, the meshes of which are larger 

 and less regular and the bars of which are more membranous, 

 having more the characters of being . branches of nucleated 

 branched-cells than, as we have seen, is the case with true adenoid 

 tissue. The meshes of this reticulum like those of adenoid tissue 

 are occupied by leucocytes ; but these are not so numerous, and 

 moreover more readily escape from this situation than from 

 the follicles, so that when a section of a fresh gland is brushed 

 with a camel's hair pencil or shaken up in normal saline solution, 

 the spaces of which we are speaking are to a large extent cleared 

 of the leucocytes previously present, while the follicular substance 

 still remains crowded with them. After treatment with silver 

 nitrate it is seen that the surface of the trabeculae (and capsule) 

 bordering this space in each alveolus is lined with sinuous 

 epithelioid plates, and a coating of similar plates may sometimes 

 be made out on the surface of the follicular substance. In other 

 words this space between the trabeculse and the follicular 

 substance is a lymph-space corresponding to the lymph s;nus of 

 the solitary follicle of the intestine, and indeed is spoken of as the 

 lymph sinus or lymph channel ; the lymph sinus of an alveolus of a 

 lymphatic gland differs from the lymph sinus of a solitary follicle of 

 the intestine in its space being much broken up by reticular 

 tissue. 



The irregular passages of the medulla are similarly occupied 

 by a central mass of follicular substance surrounded by a lymph 

 sinus; but, whereas in the alveoli the masses of follicular substance 

 take on the form of more or less pyramidal blocks, in the medulla 

 the follicular substance is arranged in the form of branching and 



