clxxviii 



BLOOD-VESSELS. 



In vessels one or two degrees larger, there is added on the primitive 

 homogeneous membrane a layer of plain muscular tissue, in form of the usual 



Fig. XCVII. 



Fig. XCVII. MAGNIFIED VIEW OP CAPILLARY VESSELS FROM THE BLADDER OF THE 



CAT. 



A, V, an artery and a vein ; ?', transitional vessel between them and cc, the capillaries. 

 The muscular coat of the larger vessels is left out in the figure to allow the epithelium to 

 be seen ; at c\ a radiate epithelium-scale with four pointed processes, running out upon 

 the four adjoining capillaries (after Chrzonszczewsky, Virch. Arch, lbb'6). 



oblong contractile fibre-cells, which are directed across the diameter of the 

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

 means of acetic acid, as shown in the figure (xcvm). This layer corre- 

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

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

 single long cell may turn spirally round the tube ; in larger vessels, espe- 

 cially those of the arterial system, they are of course more densely laid on. 

 Outside the muscular coat is the areolar or connective tissue coat, con- 

 taining fibres and connective tissue corpuscles, with longitudinally placed, 

 nuclei. 



In vessels of ^ of an inch in diameter, or even less, the elastic layers of 

 the inner coat may be discovered (fig. xcvm. , A, 3), in the form generally 

 of fenestrated membrane, more rarely of longitudinal reticulating elastic 

 fibres ; while the primitive membrane disappears. The small veins, but 

 two or three removes from the capillaries, differ from arteries of cor- 

 responding size, chiefly in the inferior development of their muscular 

 tissue. 



In reference to the structure of capillaries, it is to be further observed 

 that, in parts which are pervaded by a supporting network of retiform con- 

 nective tissue, such as the substance of the lymphatic glands, the solitary 



