178 



BLOOD-VESSELS. 



from the reticulating processes of the cells. This coating has been named by 

 His, the adventitia ca^illaris. 



In vessels one or two degrees larger (small arteries and veins), there 

 is added outside the epithelioid layer (fig. 119, a, a), a layer of plain 



Fig. 119. 



Fig. 119. — A Small Artery A, and vein V, from the Subcutaneous Connective 

 Tissue of the Rat. Treated with Nitrate of Silver. 175 Diameters. 



a, a, epithelioid cells with b, V, their nuclei ; in, m, transverse markings due to 

 staining of substance between the muscular fibre-cells ; c, c, nuclei of connective tissue 

 corpuscles attached to exterior of vessel. 



muscular tissue, in form of the usual oblong contractile fibre-cells, 

 which are directed across the length of the vessel (fig. 119, m, m). 

 The elongated nuclei of these cells may be brought into view by 

 means of acetic acid, as shown in fig. 120. This layer corresponds 

 with the middle or muscular coat of the arteries. In the smallest 

 vessels in which it appears the muscular cells are few and apart, and a 



