74 GENERAL ANATOMY. 



tance from the point at which they are distributed; they ramify in the loose 

 areolar tissue connecting the artery with its sheath, and are distributed to the 

 external and middle coats, and, according to Arnold and others, supply the in- 

 ternal coat. Minute veins serve to return the blood from these vessels ; they 

 empty themselves into the venae comites in connection with the artery. 



Arteries are also provided with nerves, which are derived chiefly from the 

 sympathetic, but partly from the cerebro-spinal system. They form intricate 

 plexuses upon the surface of the largest trunks, the smaller branches being 

 usually accompanied by single filaments ; their exact mode of distribution is 

 unknown. According to Kolliker, the majority of the arteries of the brain 

 and spinal cord, those of the choroid and of the placenta, as well as many arte- 

 ries of muscles, glands, and membranes, are unprovided with nerves. 



The Capillaries. The smaller arterial branches (excepting those of the caver- 

 nous structures of the sexual organs, and in the uterine placenta) terminate in 

 a network of vessels which pervade nearly every tissue of the body. These 

 vessels, from their minute size, are termed capillaries (capillus r "a hair"). They 

 are interposed between the smallest branches of the arteries and the commencing 

 veins, constituting a network, the branches of which maintain the same diame- 

 ter throughout, the meshes of the network being more uniform in shape and 

 size than those formed by the anastomoses of the small arteries and veins. 



The diameter of the capillaries varies in the different tissues of the body, 

 their usual size being about $ $$$ of an inch. The smallest are those of the 

 brain, and the mucous membrane of the intestines ; the largest, those of the 

 skin, and the marrow of bones. 



The form of the capillary net varies in the different tissues, being modifica- 

 tions chiefly of rounded or elongated meshes. 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 ; the meshes being more or less 

 angular, sometimes nearly quadrangular, or polygonal ; more frequently, ir- 

 regular. Elongated meshes are observed in the bundles of fibres and tubes corn- 

 posing 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 or 

 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 papillas of the tongue and skin. 



The number of the capillaries, and the size of the meshes, determines the 

 degree of vascularity of a part. The closest network, and the smallest inter- 

 spaces are found in the lungs and in the choroid coat of the eye. In the liver 

 and lung, the interspaces 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 cellular coat of 

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

 As a general rule, the more active the function of an organ is, the closer is its 

 capillary net, and the larger its supply of blood, the network being very nar- 

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

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

 gether absent in tendons and cartilages, in which very little organic change 

 occurs after their formation. 



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

 geneous membrane, in which are imbedded, at intervals, minute oval corpuscles, 

 probably the remains of the nuclei of the cells from which the vessel was 

 originally formed. 



In the largest capillaries (which ought perhaps to be described rather as the 



