CAPILLARY BLOOD-VESSELS AND LYMPHATICS 



95 



exposure that the white blood-corpuscles, which always tend to 

 pass into the inert layer, and to adhere occasionally to the inner sur- 



FIG. 118. CAPILLARY VESSELS FROM 

 THE BLADDER OF THE CAT, MAG- 

 NIFIED. 



The outlines of the cells are stained by nitrate 

 of silver. 



FIG. 119. CAPILLARY BLOOD- 

 VESSELS IX THE WEB OF A 

 FROG'S FOOT, AS SEEN WITH 

 THE MICROSCOPE. 



The arrows indicate the course of the 

 blood. 



face of the blood-vessels, here and there pass through the coats of the 

 small vessels, and appear as migratory cells in the surrounding 

 connective tissue. 



LYMPHATIC SYSTEM. 



To the lymphatic system belong not only the lymphatic vessels and 

 lymphatic glands, but also the cavities of the serous membranes, which 

 are moistened with lymph and are in open communication with the 

 lymphatic vessels in their parietes. 



The larger lymphatic vessels somewhat resemble the veins in 

 structure, except that their coats are much thinner and their valves 

 much more numerous. In lymphatics of somewhat smaller size, the 

 wall of the vessel is formed, first, by a lining of pavement-epithelium 

 cells (endotheliurn of some authors), which are elongated in the direc- 

 tion of the axis of the vessel ; and, secondly, by a layer of circularly and 

 obliquely disposed muscular fibres. In the smallest vessels (lymphatic 

 capillaries], which, however, are generally considerably larger than 

 the blood-capillaries, there is nothing but the epithelium remaining, 

 and the cells of this are frequently not more elongated in one direction 

 than in another, but have a characteristic wavy outline (fig. 121). 



Lymphatics begin in two ways either in the form of plexuses, as 

 in membranes (fig. 120), or as lacunar interstices, as is the case in 

 some of the viscera. 



