CAVITIES OF SEROUS MEMBRANES. 



333 



sinus. The afferent vessel a.l. loses all its coats, save the endo- 

 thelial, as it enters the gland, and this is continuous with that 

 lining the lymph-sinus ; the same is true of the efferent vessel, 

 which emerges at the hilum. The structure of the lymphatic 

 gland is such that the lymph in its flow passes through the pulp 

 and takes up lymphocytes. It may also deposit any poisonous 

 matter which it has absorbed, and thus prevent its entrance into 

 the blood. The arteries supplying blood to the gland enter at the 

 hilum, and the veins emerge at the same point. 



a.l. 



\ 



FIG. 178. Diagram of a lymphatic gland, showing afferent (a.l. ) and efferent 

 (e.L] lymphatic vessels ; cortical substance (C); medullary substance (M) ; fibrous 

 coat (c), sending trabeculse (tr) into the substance of the gland, where they branch, 

 and in the medullary part form a reticulum ; the trabeculse are surrounded by the 

 lymph-path or sinus (Is), which separates them from the adenoid tissue (l.h) 

 (Sharpey). 



Cavities of Serous Membanes. The serous membranes 

 are closed lymph-sacs, made up of connective tissue lined inter- 

 nally with pavement-epithelial cells, termed endothelium. Be- 

 tween some of these cells are openings stomata which are 

 surrounded by small protoplasmic cells. They are very distinct 

 in the peritoneal covering of the rabbit's diaphragm. The 

 stomata are openings into the lymphatic vessels through which 

 lymph is pumped by the contraction and dilatation of the serous 

 cavities, brought about by respiration and circulation. 



The serous membranes are : (1) peritoneum ; (2) pleura ; (3) 



