E. THE SMALL INTESTINE, BY E. VERSON. 565 



The finer structure of the parenchyma of the villi is pre- 

 cisely similar to that of the rest of the mucous membrane, 

 being composed of the tissue termed adenoid tissue by His ; 

 that is, of a plexus of anastomosing corpuscles, in the meshes 

 of which cells are contained. These characters are not, how- 

 ever, equally well marked in all classes of animals, and varia- 

 tions may even be observed to occur in one and the same species, 

 in accordance with age, the retiform tissue presenting a more 

 uniform trabecular structure, or forming a delicate plexus of 

 fibres, at the points of decussation of which a nucleus or two 

 only may be discovered, the number of cells contained in the 

 meshes having coincidently undergone considerable diminution. 

 A similar transformation of the adenoid tissue of the mucous 

 membrane may also be observed at certain points immediately 

 beneath the epithelium a circumstance which has led to the 

 admission of a separate basement membrane, situated between 

 the epithelium and the mucous membrane. No such mem- 

 brane, however, can either be isolated or shown to form a con- 

 tinuous layer. 



LYMPH FOLLICLES. At the free border of the jejunum and 

 ileum roundish or elliptical areas occur, with, in the latter case, 

 their long axes corresponding to that of the intestine, and hav- 

 ing a length of T5 centimeters, and a breadth of 7' 20 millimeters. 

 Their surface is convex, projecting into the lumen of the tube, 

 and has either a few villi scattered over it, or is altogether 

 destitute of them. These are the Peyer's patches, which, when 

 examined with low powers, or sometimes even with the naked 

 eye, appear as a group of roundish, pyriform, or more flask- 

 shaped corpuscles, the so-called follicles. These dip into the 

 submucous tissue with their rounded extremities, whilst their 

 thinner ends form projections on the free surface of the intes- 

 tinal mucous membrane, and must consequently pierce the 

 muscularis mucosse, the fasciculi of which, in point of fact, 

 separate to permit the passage of the follicles. 



A single Peyer's patch may include twenty or more such fol- 

 licles lying in close contiguity, and only separated from one 

 another by thin prolongations of the submucous tissue. The 

 inferior or deep surfaces of the follicles are somewhat flattened, 



