PERIVASCULAR SPACES AND STOMATA. 



405 



addition to it, the blood-vessels of the brain have a lymph-space within the 

 adventitia of the blood-vessels ( Virchow-Robin's space). It is partly lined by well- 

 defined endothelium. Where the blood-vessels begin to increase considerably in 



A 



Fig. 159. 



Perivascular lymphatics A, aorta 

 of tortoise ; B, artery from the 

 brain. 



Fig. 160. 



.Stomata from the great lymph-sac of a 

 frog a, stoma open; b, half-closed; 

 c, closed. 



diameter, they pass through the wall of the lymphatics, and the two vessels 

 afterwards take separate courses. In all cases, where there is a perivascular space, 

 the passage of lymph- and blood-corpuscles into the lymphatics is greatly facili- 

 tated. In the tortoise, the large blood-vessels are often surrounded with, peri- 

 vascular lymphatics. Fig. 159, A, gives a representation of the aorta sur- 

 rounded by a perivascular space (Gegenbaur) which is visible to the unaided eye. 

 In mammals, the perivascular spaces are microscopic. 



(4.) Origin in the form of interstitial slits within organs. Within the testis, the 

 lymphatics begin simply in the form of numerous slits, which occur between the 

 coils and twists of the seminal tubules. They take the form of elongated spaces 

 bounded by the curved cylindrical surfaces of the tubules. The surfaces, however, 

 are covered with endothelium. The lymphatics of the testis get independent 

 walls after they leave the parenchyma of the organ. In many other glands, the 

 gland-substance is similarly surrounded by a lymph-space. The blood-vessels 

 pour the lymph into these spaces and from them the secreting cells obtain the 

 materials necessary for the formation of their secretion. 



(5. ) Origin by means of free stomata on the walls of the larger serous cavities 

 (Fig. 160, a). The investigations of v. Recklinghausen, Ludwig, Dybkowsky, 

 Schweigger-Seidel, Dogiel, and others have shown, that the old view of Mascagni, 

 that the serous cavities freely communicate with the lymphatics, is correct. The 

 investigation of the serous surfaces, most easily accomplished on the septum of 

 the great abdominal lymph-sac of the frog, by means of silver nitrate, reveals 

 the presence of relatively large free openings or stomata lying between the 

 endothelium. Each stoma is bounded by several cells, which have a granular 

 appearance, and are capable of undergoing a change of shape, so that the size 

 of the stoma depends upon the degree of contraction of these cells; thus the 

 stoma may be open (a), half open (b), or completely closed (c). These stomata 

 are the origin of the lymphatics. The serous cavities belong therefore to the 

 lymphatic system, and fluids placed in the serous cavities readily pass into the 



