500 



ANGIOLOGY 





Elongated meshes are observed in the muscles and nerves, the meshes resembling parallelograms 

 in form, the long axis of the mesh running parallel with the long axis of the nerve or muscle. 

 Sometimes the capillaries have a looped arrangement; a single vessel projecting from the common 

 net-work and returning after forming one or more loops, as in the papillae of the tongue ar.d 

 skin. 



The number of the capillaries and the size of the meshes determine the degree of vascularily 

 of a part. The closest network and the smallest interspaces are found in the lungs and in the 

 choroid coat of the eye. In these situations the interspaces are smaller than the capillary vessels 

 themselves. In the intertubular plexus of the kidney, in the conjunctiva, and in the cutis, the 

 interspaces are from three to four times as large as the capillaries which form them; and in the 

 brain from eight to ten times as large as the capillaries in their long diameters, and from four 

 to six times as large in their transverse diameters. In the adventitia of arteries the width of the 

 meshes is ten times that of the capillary vessels. As a general rule, the more active the func- 

 tion of the organ, the closer is its capillary net and the larger its supply of blood; the meshes of 

 the network are very narrow in all growing parts, in the glands, and in the mucous membranes, 

 wider in bones and ligaments which are comparatively inactive; bloodvessels are nearly alto- 

 gether absent in tendons, in which very little organic change occurs after their formation. In 

 the liver the capillaries take a more or less radial course toward the intralobular vein, and their 

 walls are incomplete, so that the blood comes into direct contact with the liver cells. These 

 vessels in the liver are not true capillaries but "sinusoids;" they are developed by the growth 

 of columns of liver cells into the blood spaces of the embryonic organ. 



Endothelial and sub- 

 endothelial layer of 

 . _ _ _ inner coat 



Elastic layer 



I Innermost layers of 

 middle coat 



Outermost layers of 

 middle coat 



Innermost part of 

 outer coat 



FIG. 450. Section of a medium-sized artery. (After Griinstein.) 



Structure. The wall of a capillary consists of a fine transparent endothelial layer, composed 

 of cells joined edge to edge by an interstitial cement substance, and continuous with the endo- 

 thelial cells which line the arteries and veins. When stained with nitrate of silver the edges which 

 bound the epithelial cells are brought into view (Fig. 451). These cells are of large size and of 

 an irregular polygonal or lanceolate shape, each containing an oval nucleus which may be dis- 

 played by carmine or hematoxylin. Between their edges, at various points of their meeting, 

 roundish dark spots are sometimes seen, which have been described as stomata. though they are 

 closed by intercellular substance. They have been believed to be the situations through which 

 the colorless corpuscles of the blood, when migrating from the bloodvessels, emerge; but this 

 view, though probable, is not universally accepted. 



Kolossow describes these cells as having a rather more complex structure. He states that 



