366 



MANUAL OF HISTOLOGY. 



muscular elements as middle coat, and finally (d) the external envelope 

 of connective- tissue. 



Vessels of this kind can by no means be called any longer capillaries ; 



d- 



Fig. 360. Two considerable vessels from the pia mater of the human brain. 

 1. A small arterial twig. 2. A venous twig; a, 6, internal; c. middle; 

 and d, external layer. 



they bear from henceforth far more the character of fine arterial and 



venous branches. According to 

 their nature in this respect, they 

 offer certain differences for our 

 consideration, and besides, a series 

 of others of a more local or indi- 

 vidual kind. 



Taking vessels of about 0'0282- 

 0-04512mm.indiameter(fig.360), 

 only two membranes are to be dis- 

 tinguished in a venous branch (2) 

 of this kind. In the first place an 

 inner (a, b), in the form of a toler- 

 ably resistant elastic tunic, re- 

 markable for its tendency to form 

 smaller or larger longitudinal 

 folds, and studded with numerous 

 nuclei. The latter on treatment 

 with silver are seen to be the 

 nuclear formations of the vascular 

 cells, which are smaller here than 

 in the capillaries, presenting also 

 a broader and more rhomboidal 

 figure. It is still a matter of 



uncertainty whether these are again clothed in a thin longitudinally marked 



Fig. 361. An artenal branch. At (ft) the homo- 

 geneous internal layer destitute of nuclei; (c) mid- 

 dle tunic formed of contractile fibre cells; d, the 

 external connective-tissue tunic. 



