76 VESSELS. ARTERIES. VEINS. CAPILLARIES, ETC. 



that of the arteries ; but it is to be noticed that in 

 certain veins, principally in those belonging to the 

 portal system, muscular fibres have been found in its 

 deeper portion, longitudinal in their direction. The 

 presence of these muscular fibres, and the direction of 

 their course, explains the reason why these veins 

 diminish in length under the influence of the stimulus 

 of galvanism. 



ize. In veins of the smallest size the three tunics are 

 still distinguishable ; the innermost is a simple epi- 

 thelial layer ; sometimes, however, there is a fenes- 

 trated layer outside of it, presenting exceedingly 

 delicate meshes (PI. XVI. fig. V. 6). The remaining 

 tunics resemble exactly those of arteries of similar 

 calibre (fig. V. 1, 3). 



valves. The valves of veins are formed by their internal 

 coat. A lamina of pavement-epithelium constitutes 

 their surface (PL XVI. fig. III. 1). More deeply, we 

 encounter wavy and parallel fasciculi of connective 

 fibres, and a web of delicate elastic fibres intermingled 

 with plasmatic cells. The latter are rendered visible 

 by the addition of dilute acetic acid, which dissolves 

 the connective fibres. 



The vasd vcisorum are arterioles and veinules. 

 According to Kolliker, they are to be found on ves- 

 sels even of the smallest calibre (of the diameter of 

 one half a line and less) ; they are distributed mainly 

 to the outer coat ; in the middle coat there are a few, 

 but in the inner coat I have never seen them. The 



Nerves, nerves which supply the walls of vessels are few in 

 number, and those which .are encountered are for the 

 most part destined to the organs supplied by the 



