1130 GENERAL ANATOMY OR HISTOLOGY. 



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 epithelial cells are brought 

 into view (Fig. 668). These cells are of large size and of an irregular polygonal or 

 lanceolate shape, each containing an oval nucleus which may be brought into view 

 by carmine or hsematoxylin. Between their edges, at various points of their 

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

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

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

 migrating from the blood-vessels, emerge ; but this view, though probable, is not 

 universally accepted. 



Kolossow, a Russian observer, describes these cells as having a rather more 

 complex structure. He states that they consist of two parts : of hyaline ground- 

 plates, and of a protoplasmic granular part, in which is imbedded the nucleus, on 

 the outside of the ground-plates. The hyaline internal coat of the capillaries 

 does not form a complete membrane, but consists of "plates" which are inelastic, 

 and, though in contact with each other, are not continuous ; when, therefore, the 

 capillaries are subjected to intra- vascular pressure, the plates become separated 

 from each other ; the protoplasmic portions of the cells, on the other hand, are 

 united together. 



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

 nective-tissue cells is found around the simple capillary tube, particularly in the 

 larger ones ; and in other places, especially in the glands, the capillaries are 

 invested with retiform connective tissue. 



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

 smallest arteries or pre-capillaries) there is, outside the epithelial layer, a muscular 

 layer, consisting of contractile fibre-cells, arranged transversely, as in the tunica 

 media of the larger arteries (Fig. 669). 



The veins, like the arteries, are composed of three coats internal, middle, and 

 external ; and these coats are; with the necessary modifications, analogous to the 

 coats of the arteries ; the internal being the endothelial, the middle the muscular, 

 and the external the connective or areolar (Fig. 670). The main difference be- 



Endothelial and 

 S subendothelial 



layers. 

 Elastic layer. 



t- Middle coat. 



U 



ir- -Outer coat. 



FIG. 670. Transverse section of part of the wall of one of the posterior tibial veins. (After Sehiifer). 



tween the veins and the arteries is the comparative weakness of the middle coat of 

 the former, and to this is due the fact that the veins do not stand open when 

 divided, as the arteries do, and that they are passive rather than active organs of 

 the circulation. 



In the veins immediately above the capillaries the three coats are hardly to be 

 distinguished. The endothelium is supported on an outer membrane separable-into 

 two layers, the outer of which is the thicker, and consists of a delicate, nucleated 

 membrane (adventitia), while the inner is composed of a network of longitudinal 

 elastic fibres (media). In the veins next above these in size (one-fifth of a line, 

 according to Kolliker) a muscular layer and a layer of circular fibres can be traced, 

 forming the middle coat, while the elastic and connective elements of the outer 

 coat become more distinctly perceptible. In the middle-sized veins the typical 

 structure of these vessels becomes clear. The endothelium is of the same character 



