STRUCTURE AND PROPERTIES OF THE BLOOD-VESSELS. 



anastomosing fibrils derived from non-medullated nerves terminate by small 

 end-plates in the capillary walls. Ganglia in communication with the nerves of 

 capillary vessels are found only in the distribution of the sympathetic nerves. 

 The minute blood-vessels that communicate directly with the capillaries possess, 

 in addition to endothelium, an entirely structureless investing membrane. 



The Veins differ from the arteries in the main in the fact that they have a 

 larger caliber than the corresponding arteries and thinner walls on account of 

 the much feebler development of the elastic and muscular elements. Among the 

 latter longitudinal fibers are much more commonly found than transverse. Veins 

 are also distinctly more distensible with the same degree of traction. The adven- 

 titia is as a rule relatively the thickest coat. The presence of valves is limited 

 to certain areas of the body. 



The intinia or internal coat is provided with short endothelial cells, beneath 

 which, in the smallest veins, is a structureless layer, which in the somewhat larger 

 vessels is composed principally of longitudinal elastic fibers (always thinner than 

 in the arteries). In the large veins this layer may assume the character of a 

 fenestrated membrane, which here and there in the femoral and iliac veins is even 

 duplicated. It is held together by a delicate connective tissue containing 

 spindle-cells. The intima in 

 the femoral and popliteal veins 

 contains a few scattered muscle- 

 fibers. 



The media or middle coat in 

 the larger veins is constituted 

 of alternate layers of elastic and 

 muscular elements, with a fairly 

 abundant fibrillar connective 

 tissue. The media is always 

 thinner, however, than in the 

 corresponding arteries. The 

 number of these alternating 

 layers becomes progressively 

 smaller in the following veins, 

 in the order of their enumera- 

 tion: popliteal vein, veins of 

 the lower extremity, veins of 

 the upper extremity, superior 

 mesenteric, the remaining veins 

 of the abdominal cavity, the 

 hepatic, pulmonary, and coro- 

 nary veins. The following veins 

 are altogether devoid of mus- 

 cle-tissue: the veins of bones, 

 muscles, the central nervous 

 system and its membranes, the 

 retinal veins, the superior cava 

 with the large trunks that empty into it, and the upper portion of the inferior 

 cava. In these vessels the media is much more feebly developed. In the smallest 

 veins the media consists merely of a delicate fibrillar connective tissue in which a 

 few scattered longitudinal and transverse unstriated muscle-cells make their ap- 

 pearance as the center of the circulation is approached. 



The adventitia or external coat of the veins is, generally speaking, thicker than 

 that of the corresponding arteries. It always contains more abundant connective 

 tissue, usually consisting of longitudinal fibers, and on the other hand fewer large- 

 meshed networks of elastic elements. Some veins, however, contain also longi- 

 tudinal muscle fibers: the renal vein, the portal vein, the inferior cava in the 

 hepatic region, the veins of the lower extremity. The valves consist of finely 

 fibrillated connective tissue in which stellate cells are embedded; the convex 

 surface of the valves is covered with a network of elastic fibers, and both surfaces 

 are invested with endothelium. The valves contain many muscle-fibers. 



The sinuses of the dura mater are spaces lined with endothelium between 

 duplicatures, or cleft-like invaginations of this membrane. 



Cavernous spaces may be regarded as having been produced by numerous 

 divisions and anastomose's of fairly large veins of unequal size, closely following 

 one another. The vessel-wall frequently appears cribriform or like a sponge the 



FIG. 40. Capillary Vessels, the Boundaries of the Cells (Cement- 

 substance between the Endothelial Cells) have been Stained 

 Black with Silver Nitrate and the Nuclei of the Endothelial 

 Cells Made Prominent by Staining. 



