266 MANUAL OF HISTOLOGY. 



entire diaphragm, with the mediastinal portion, together with 

 the heart and pericardium. 



It will be seen that the endothelium is composed of a great 

 number of polygonal plates whose edges are glued together by 

 a substance which has been stained brown or black by the sil- 

 ver. Nuclei are seen in many of them, or they can be shown 

 by staining with carmine or hsematoxylon. Small openings 

 are to be seen in certain localities surrounded by cells of a 

 more cubical form, with large, distinct nuclei. In other places 

 small dark spots are seen between the cells. The openings are 

 known as stomata, and communicate with lymphatic vessels 

 running beneath the endothelium. The dark spots &YQ pseudo- 

 stomata, and are similar in their nature to the pseudo-stomata 

 of the alveoli and bronchial mucous membrane ; i.e., they are 

 the ends of processes of the branched cells of the ground-sub- 

 stance reaching up between the endothelial plates. In order 

 to demonstrate the ground- sub stance or connective-tissue layer 

 of the pleura, the fresh surface is carefully pencilled with a 

 soft brush dipped in the fluid of the abdominal cavity, or in 

 artificial serum. After washing with distilled water, the solu- 

 tion of silver nitrate is poured over it, and it is treated as be- 

 fore. On examination, the branched connective-tissue cells 

 are seen communicating with each other by their processes. 



Blood-vessels and lymphatics are also seen, and in a favor- 

 able place the endothelium of the latter is seen to be continuous 

 with the branched cells. These cells line the cavities of the 

 connective tissue, and belong to the lymph lacunar system. 



The lymphatic vessels accompany the blood-vessels, some- 

 times ensheathing them. They are identified by the shape of 

 their endothelial cells, which are wider and more polygonal in 

 form than those of the veins. It will be seen from this descrip- 

 tion that the serous membrane is a lymphatic structure. Its 

 cavity communicates by means of the stomata with the lym- 

 phatic vessels below, while, by means of the pseudo-stomata, 

 it communicates with the lacunar spaces which are lined by 

 the branched cells. To demonstrate the pulmonary pleura, 

 the lungs should be excised, moderately distended with air 

 (which is retained in them by ligature of the trachea), treated 

 with silver nitrate, as already described, and then immersed in 

 alcohol. After a few days sections are made parallel to the 

 surface. 



