138 MICROSCOPIC ANATOMY o/-' 77/2? ORGANS. 



wliich is repeated many times to make up the organ. It corre- 

 sponds also in this case with the cellular unit which is repre- 

 sented by the follicle. 



(r) Peripheral Lymph Nodules. 



Collections of lymphoid tissue are present in many organs 

 in the form of single follicles, or many of these together. They 

 are not so definitely connected with the lymph-vessels as the 

 true lymph glands are. They may occur merely as a diffuse 

 infiltration of the tissue by lymphocytes. The so-called solitary 

 follicles of the alimentary canal are definite well-circumscribed 

 masses of lymphocytes, possessing germinal centres, and all the 

 characters of lymph follicles. Collections of these follicles are 

 seen in the Peyer's patches of the small intestine. 



3. SPLEEN. 



In the spleen, as in the lymph gland, we can distinguish a 

 connective-tissue capsule, sending processes down into the organ 

 to form its framework ; and adenoid tissue contained in the 

 framework, the so-called spleen pulp. 



The capsule and the trabeculse proceeding from it are made 

 up of connective tissue with a considerable number of smooth 

 muscle cells and elastic fibres. They can be easily distinguished 

 from the corresponding structures in the lymph gland by their 

 strong, coarse appearance. They are nearly always thicker, and 

 the muscle cells give to them a less finely fibrous character. At 

 the hilus the trabeculse and capsule form a sheath for the blood- 

 vessels which enter there. This sheath retains its firm fibrous 

 character throughout the course of the veins, but with the 

 arteries it gives place to a fine reticular tissue when these 

 vessels by branching have become as small as 0.25 mm. in 

 diameter. This reticular tissue contains in its meshes large 

 quantities of lymphocytes. 



The arterial sheath, consisting of adenoid tissue, forms, in 

 some animals, a continuous layer around the vessel wall. In 

 other animals, however, it is gathered into spherical or ovoid 

 masses which resemble the follicles of the lymph gland. These 



