THE VASCULAR SYSTEM. 



83 



common network and returning after forming one or more loops, as in the 

 papillae of the tongue and skin. The number of the capillaries, and the size of the 

 meshes, determine the degree of vascularity 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 them- 

 selves. In 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 diameter, and 

 from four to six times as large in their transverse diameter. In the adventitia 

 of arteries the width of the meshes is ten times that of the capillary vessels. 

 A- a general rule, the more active the function of the organ, the closer is its 

 capillary net and the larger its supply of blood ; the network being very narrow in 

 all growing parts, in the glands, and in the mucous membranes ; wider in bones 

 and ligaments, which are comparatively inactive ; and nearly altogether absent in 

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



Structure. The walls of the capillaries consist of a fine, transparent, endothelial 

 layer, composed of cells joined edge to edge by an interstitial cement-substance, 

 and continuous with the endothelial cells which line the arteries and veins. "When 

 stained with nitrate of silver the edges which bound the endothelial cells are 

 brought into view (Fig. 59). These cells are of large size and of an irregular polyg- 

 onal or lanceolate shape, each containing an oval nucleus which may be brought 

 into view by carmine or haematoxylin. Between their edges, at various points of 

 their meeting, roundish dark spots are sometimes seen, which have been described as 



FIG. 59. Capillaries from the 

 mesentery of a guinea-pig after treat- 

 ment with solution of nitrate of sil- 

 ver, a. Cells. 6. Their nuclei. 



FIG. 60. Finest vessels on the arterial side. From the human 

 brain. Magnified 300 times. 1. Smallest artery. 2. Transition 

 vessel. 3. Coarser capillaries. 4. Finer capillaries, a. Structure- 

 less membrane still with some nuclei, representative of the 

 tunica adventitia. b. Nuclei of the muscular fibre-cells, c. 

 nuclei within the small artery, perhaps appertaining to an 

 endothelium. d. Nuclei in the transition vessels. 



stomata, though they are closed by intercellular substance. They have been 

 believed to be the situation through Vhich the white corpuscles of the blood, when 

 migrating through the blood-vessels, emerge ; but this view, though probable, is 

 not universally accepted. 



In many situations a delicate sheath or envelope of branched nucleated connec- 



