130 



STRUCTURE AND PROPERTIES OF THE BLOOD-VESSELS. 



smaller arteries, the number of unstriated muscle-fibers increases progressively 

 until they form a strong layer of circular muscle-fibers with almost complete dis- 

 appearance of the connecting substance. The outer elastic layer forms the bound- 

 ary between the media and the adventitia. In the large arteries the connecting 

 substance greatly predominates over all other tissues: Separated by layers of 

 delicate fibrous tissue there are numerous (as many as 50) thick, elastic, fibrillated 

 or fenestrated membranes arranged in concentric layers and chiefly in the trans- 

 verse direction. Scattered here and there between these membranes are occa- 

 sional smooth muscle-cells arranged transversely, less commonly obliquely, or 

 longitudinally. 



The initial portions of the aorta and pulmonary artery, the arteries in bones 

 and the retinal arteries are devoid of muscle-tissue. The descending aorta and 

 the common iliac and popliteal arteries possess oblique and longitudinal muscle- 

 fibers lying among the transverse fibers. The renal, splenic and internal sper- 

 matic arteries contain longitudinal bundles at 

 the inner surface of the media; the umbilical 

 arteries, which are exceedingly rich in muscle- 

 tissue, contain longitudinal bundles both on 

 the inner and on the outer surface. 



The external or adventitious coat in the 

 smaller arteries is a delicate, structureless 

 membrane containing a few protoplasmic 

 cells. In somewhat larger vessels there is an 

 additional layer of elastic tissue of delicate 

 fibers containing strands of fibrillated con- 

 nective tissue (d) . In the medium-sized and 

 largest arteries the greater part of the ad- 

 ventitia consists of bundles of fibrillated 

 connective tissue containing connective-tissue 

 cells, and not infrequently an admixture of 

 fat-cells, running obliquely and crossing each 

 other at numerous points. Among them and 

 chiefly toward the media are found fibrous or 

 fenestrated elastic layers. At the boundary 

 between the adventitia and the media the 

 elastic elements in the smaller and medium- 

 sized arteries fuse to form a more indepen- 

 dent elastic membrane (Henle's outer elastic 

 membrane). Longitudinal unstriated mus- 

 cle-fibers in scattered bundles are found in 

 the adventitia of the arteries of the penis, of 

 the descending aorta, the renal, splenic, in- 

 ternal spermatic, iliac, hypogastric, and 

 superior mesenteric arteries. 



Bonnett suggests the following natural 

 division of the layers of the arterial wall: 

 i . The intima embraces the endothelial tube 

 and the tissues as far as the inner elastic 

 layer. 2 . The media contains all those parts 

 that are situated between the inner and the 

 outer elastic layer. 3. The adventitia includes the layers found to the outer side of 

 the elastic membrane. 



The Capillaries, which undergo frequent division without suffering diminu- 

 tion in caliber, and in their subsequent course unite again, have diameters 

 varying from 5 to 6 fj, (retina, muscles) to from 10 to 20 // (bone-marrow, liver, 

 choroid) The tubes are formed of a single layer of nucleated endothelial cells, 

 with protoplasmic cell-bodies, which in the smaller tubes are spindle-shaped 

 and in the larger vessels are more polygonal (as is the case with the cells ot 

 serous cavities); they are connected by numerous intercellular bridges in the 

 depths of the cell-substance (like epithelial cells). The boundaries of the cells 

 are demonstrable as black lines by injection of a solution of silver nitrate 

 stained cement-substance exhibits in some places intercalated areas of larger 

 size. Whether these are to be regarded as true openings or stomata, through 

 which it is possible for red and white cells to escape, or merely as denser aggre- 

 gations of the stained cement-substance is still an undecided question. Delicate 



FIG. 39. Small Arterial Twig Showing the In- 

 dividual Layers of the Arterial Wall: a, 

 endothelium; b, elastic inner coat; c, 

 layer of circular muscle-fibers; d, con- 

 nective-tissue adventitia. 



