STEUCTURE OF SMALLER ARTERIES. 



177 



Fisr. 118. 



of the brain and spinal cord. On the other hand, it has wide meshes 

 and comparatively few vessels in the ligaments, tendons, and otlier 

 allied textnres. In infants and young persons, the tissues are more 

 vascular than in after-life ; growing parts, too, are more abundantly 

 supplied with vessels than those which are stationary. 



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 (tig. 116). In other parts the meshes are rounded or jDoly- 

 gonal, with no one dimension greatly predominating (fig. 117). In 

 the smaller-sized j^apillie of the skin and mucous membranes, the 

 vessels of tlie network are often drawn out into prominent loops. 



Structure of the Smaller-sized Vessels and Capillaries. — The 

 smallest arteries and veins pass by gradual transition into the capil- 

 lary vessels, and their finest offsets approach very near to these in struc- 

 ture ; they may therefore be con- 

 veniently considered together. 



The wall of the capillaries proper 

 is formed entirely of a simple 

 epithelioid layer, composed of flat- 

 tened lanceolate cells joined edge to 

 edge, and continuous with the cor- 

 responding layer which lines the 

 arteries and veins. The outlines of 

 the cells or their lines of junction 

 one with another may be made ap- 

 parent by nitrate of silver ; after 

 which the nuclei may be brought 

 into view by logwood or carmine 

 (fig. 118). Commonly there are not 

 more than two or three such cells 

 in the cross section of a capillary, 

 but there may be four or five. At 

 the points of junction of the capil- 

 laries the cells are much 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. 118 c). 



Fig. 118. — Capillary Vessels from 

 Bladder of Cat magnified (after 

 Clirzonszczewsky. ) 



The outlines of the cells are stained by 

 nitrate of silver. 



A protrusion of processes from the capillary waU has been observed, not only 

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

 in the fully -developed capillaries of the frog's hyaloid membrane (Strieker) : 

 for this and other reasons the cells which compose these vessels are regarded 

 as of a protoiDlasmic nature. 



Branched cells of the surrounding connective tissue are sometimes found con- 

 nected more or less intimately with those forming the capillary wall ; this is 

 more especially the case in parts which are pervaded by a supporting network of 

 retiform 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 capiUaiies may even obtain a continuous covering 



VOL. II. N 



