INTERNAL EESTITUTIVE SECRETIONS 



267 



protrusion of the villi. As shown in Fig. 87, they consist of a 

 special epithelium which encloses a lymphoid tissue : in the centre 

 of this is a lacteal, the wall of which consists of simple epithelioid 

 cells with wavy outlines, which in all probability have little stomata 



Tad. 



Tad 



FIG. 89. Cross-section through villus : A, of dog ; B, of rabbit. (Heidenhain.) C, central lacteal 

 T.ad, adenoid tissue ; E, epithelium with striated cuticle. 



or spaces at the junction of the cement substance. The central 

 lymphatic (which sometimes consists of two vessels joined at the 

 end by a loop, as shown in Fig. 88) communicates with the sub- 

 jacent interrnuscular lymph plexus. The adenoid tissue of the 

 villi is generally more developed in carnivora than in herbivora, 

 in which, on the other hand, the central lymphatic is much wider 



FIG. 90. Magnified blood-vessels of intestinal villi. (Sharpey.) From specimen injected by 

 Lieberkiihn. Each villus shows a small artery and vein with capillary network between. 



(Fig. 89). The adenoid tissue of the villi is also traversed by a 

 vascular loop consisting of small arteries and veins, united by a 

 capillary network (Fig. 90). The retiform adenoid cells are inter- 

 mixed with muscular tissue which is a prolongation of the 

 muscularis mucosae (Brlicke), and the meshes or lacunae which 

 they form contain numerous lymphocytes, filled with granules, 

 which stain black with osmic acid, but do not consist of fat, since 



