370 



BLOOD-VESSELS. 



and other allied textures. In infants and young persons, the tissues are compara- 

 tively more vascular than in after-life. 



The figure of the capillary network is not the same in all textures. In many 

 cases the shape of the meshes seems accommodated to the arrangement of the 

 elements of the tissue in which they lie. Thus in muscle, nerve, and tendon, the 

 meshes are long and comparatively narrow, and run conformably with the fibres 

 and fasciculi of these textures. In other parts, as in the lungs, in fat, and in 

 secreting glands, the meshes are rounded or polygonal, with no one dimension 

 greatly predominating. In the papillae of the skin and mucous membranes, the 

 vessels of the network are often drawn out into prominent simple or ramified loops. 



The smallest arteries and veins pass by gradual transition into the capillary 

 vessels, and their finest offsets approach very near to these in structure ; these may 

 therefore be conveniently considered along with the capillary vessels. 



Structure of the capillaries. The wall of the capillaries proper is formed 

 entirely of a simple epithelial layer, composed of flattened lanceolate cells joined 

 edge to edge, and continuous with the corresponding layer which lines the arteries 



Fig. 428. CAPILLARY VESSELS FROM THE BLADDER OF THE CAT, 

 MAGNIFIED (after Chrzonszczewsky). 



The outlines of the cells are stained by nitrate of silver. 



and veins. The outlines of the cells or their lines of 

 junction one with another may be made apparent by 

 nitrate of silver (fig. 428) ; while the nuclei, which 

 show a well-marked network of karyoplasm, may be 

 brought into view by logwood or carmine. Commonly 

 there are not more than two or three such cells in the 

 cross section of a capillary. At the points of junc- 

 tion of the capillaries the cells are usually broader 

 and not spindle-shaped, but radiate, with three or 

 four pointed branches fitting in between the cells of 

 the three or four adjoining vessels which meet at the 

 spot (fig. 428, c, c, c'). 



In capillaries which, have been submitted to the action of nitrate of silver, there is here 

 and there to be seen between the cells of the capillary wall an increase in amount of the inter- 

 cellular substance, appearing as an enlargement of the fine line of the silver deposit. To these 

 gaps in the capillary wall, which however are closed by intercellular substance, J. Arnold has 

 applied the term " stigmata ; " they are analogous to the " pseudo-stomata " found between 

 the epithelium-cells of a serous membrane. It is probable that the white blood corpuscles, 

 when migrating from the blood-vessels, pass between the epithelium-cells, especially in the 

 situation of the stigmata. 



Branched cells of the surrounding areolar tissue are found connected intimately with the 

 cells forming the capillary wall. This connection occurs almost everywhere, but it is more 

 obvious in parts which are pervaded by a supporting network of retif orm connective tissue, 

 such as the substance of the lymphatic glands, the solitary and agminated intestinal glands 

 and adjacent mucous membrane, where the small vessels and capillaries may even obtain a 

 continuous covering from the reticulating processes of the cells. This coating has been named 

 by His, adrentitia capillarts. 



Outgrowths from the capillary wall have been described by Strieker as occurring not only 

 in the progress of development, in the manner to be afterwards detailed, but also in the fully 

 developed capillaries of the frog ; and contraction both of the whole capillary wall and also of 

 the individual cells of young capillary vessels has been described (Strieker, Tarchanoff), 

 but it is not known whether the walls of the capillaries of the adult mammal possess any 

 appreciable contractility. 



Structure of the small arteries and veins. In vessels a little larger 

 than the capillaries, there is added outside the epithelial layer, a layer of plain 



