THE VASCULAR SYSTEM. 



1129 



the mucous membranes of the intestines ; and the largest those of the skin and 

 the marrow of bone, where they are stated to be as large as y^ of an inch. 



The form of the capillary net varies in the diiferent tissues, the meshes being 

 generally rounded or elongated. The rounded form of mesh is most common, and 

 prevails where there is a dense network, as in the lungs, in most glands and 

 mucous membranes, and in the cutis ; here the meshes are more or less angular, 

 sometimes nearly quadrangular or polygonal ; or more often irregular and not of 

 an absolutely circular outline. 



Elongated meshes are observed in the muscles and nerves, the meshes being 

 usually of a parallelogram form, the long axis of the mesh running parallel with 

 the long axis of the nerve and fibre. Sometimes the capillaries have a looped 

 arrangement; a single vessel projecting from the 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 

 yascularity 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 inter- 



FIG. 668.- Capillaries from the 

 mesentery of a guinea-pie after treat- 

 ment with solution of nitrate of sil- 

 ver, o. Cells. 6. Their nuclei. 



FIG. 669. 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 adventltla. b. Nuclei of the muscular fibre-cells, c. 

 nuclei within the small artery, perhaps appertaining to an 

 endothelium. d. Nuclei in the transition vessels. 



spaces are smaller than the capillary vessels themselves. 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. As 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 meshes of 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 



