THE BLOOD-VESSELS. 



91 



More frequently, there are capillaries where the endothelial 

 tube is surrounded by a connective-tissue layer, a so-called 

 adventitia capillaris. The latter is, 

 for a certainty, the primary rudi- 

 ment of the layer, which with in- 

 creasing complexity occurs in all the 

 larger vessels as the most external 

 layer or adventitia. We here meet 

 at first with either ordinary connec- 

 tive tissue, which has indeed re- 

 mained at an earlier stage, with lon- 

 gitudinally arranged nuclei or cell 

 remains (Fig. 89, d), or, when capil- 

 laries of lymphoid organs are con- 

 cerned (Fig. 88, b), the reticular con- 

 nective substance has become spread 

 over the endothelial tube in an ele- 

 gant manner, and the capillary is 

 kept distended by this cellular reticu- 

 lum, like the embroidery in a frame. 



Passing, now, to somewhat larger trunks, numerous 

 variations of the structure occur. They coincide in part 

 with the nature of the vessel, whether arterial or venous 

 branches ; they are also, in part, of a more individual or local 

 nature. 



Frequently, when we follow the capillaries towards the 

 arterial tubes, we perceive branches where a layer, striking 

 the eye by its transversely arranged nuclei (Fig. 86, 3, b), is 

 met with around the endothelial tube (a). The former con- 

 stitutes the very commencement of the muscular middle layer 

 or tunica media of the vessels. An equally large venous 

 branch usually has in the place of the latter layer a con- 

 nective-tissue adventitia. Still, it often enough occurs in 

 the finer arterial branches also, spread out over the muscular 

 layer. 



Let us take a small arterial trunk, after the manner of our 

 Fig. 89. The endothelial tube is not drawn here. Lying on 



Fig. 88. — Capillary vessels and fine 

 branches of the mammalia ; a, capil- 

 lary vessel from the brain ; />, from a 

 lymphatic gland : c, a somewhat 

 larger branch with a lymph-sheath 

 from the small intestine ; and d, a 

 transverse section of a small artery 

 of a lymphatic gland. 



