506 THE TISSUES. 



layer is first added to the capillary membrane, and which gradually 

 combines with the latter,' while the nuclei of the capillaries become 

 more closely approximated. In vessels of jjoo f an inch, they 

 have become so numerous as clearly to represent the epithelium, 

 and now the external layer has received also the addition of a nu- 

 cleated lamina, the tunica adventitia, and the vessel has become a 

 vein. (Fig. 337, C.) 



The older anatomists have also assumed the existence of still 

 finer vessels than the capillaries, never admitting the blood-corpus- 

 cles, called vasa serosa; and recently Hyrtl has admitted their ex- 

 istence in the cornea. If any such vessels exist in the cornea, they 

 must be regarded as atrophied capillaries ; the latter having formed 

 an abundant plexus in the foetus. It cannot be admitted that such 

 vessels exist in the adult in any other part, at any rate (p. 281). 

 The finest capillaries above-mentioned have a less diameter than 

 the smallest blood-corpuscles; but the latter easily adapt themselves 

 by their extensibility and elasticity to traverse the former. 



C. The Veins. 



The veins also present three tunics the adventitia (external), the 

 media, and the intima ; and may also be divided into the small, the 

 medium-sized, and the large. Their walls are always thinner than 

 those of the arteries, there being less of both the muscular and 

 the elastic element. The external coat is usually the thickest of 

 the three ; its relative and absolute thickness usually increasing 

 with the size of the vessel. The inner coat is often not thicker in 

 large veins than in those of medium size. 



1. The smallest veins may be said to consist of a nucleated indis- 

 tinctly fibrous or homogeneous areolar tissue, lined by a scaly 

 epithelium. A muscular membrane, and generally a layer of an- 

 nular fibres are first seen in veins above g J^ of an inch in diameter; 

 the contractile cells being at first oval, placed transversely (Fig. 

 337, c), and widely apart, but which afterwards become longer and 

 more numerous; and, finally, in vessels of TsW up to T ^ of an 

 inch, constituting a continuous layer, but always less developed than 

 the middle coat of the small arteries. Afterwards, elastic networks 

 gradually make their appearance in this muscular layer, and in the 

 external coat; the muscular layers also multiplying, and admitting 



areolar tissue among their elements. 





