VESSELS. ARTERIES. VEINS. CAPILLARIES, ETC. 75 



form fibres of non-striated muscle, indicating that, in 

 these little arterioles, this tissue remains permanently 

 in an imperfectly developed condition (PL VII. 2, 3). 

 As for the inner coat, it is reduced to a mere layer of 

 epithelial cells (fig. III. 4). 



SECT. II. VEINS. The structure of the veins fol- 

 lows the same general plan as that of the arteries. 

 Like them, they have three coats. The epithelial 

 layer of the internal coat is identical in every respect internal coat 

 with that of the arteries. In almost all the specimens 

 which I have examined, a fenestrated membrane has 

 been present, showing numerous openings, surrounded 

 by an interlacement of large elastic fibres (PI. XVI. 

 fig. IV. 1, 2). 



Beneath this is a third layer of fine elastic fibres, 

 forming a somewhat looser web than the correspond- 

 ing layer of the inner coat of an artery, and pene- 

 trating, by its deepest fibres, the surface of the middle 

 coat so that the dividing line between these two 

 coats is not so distinct and clear as in the walls of an 

 artery (PL XVI. fig. II. 2). 



The middle tunic presents an intermixture of Middle coat 

 elastic and muscular fibres, but the latter are not 

 uniformly distributed (fig. II. 5, 6). Their direction 

 is generally transverse, but nevertheless, near its 

 outer surface, there are some which run parallel with 

 the axis of the vessel (fig. II. 7, 8). May not this 

 unequal distribution of muscular fibre in the walls of 

 veins account for the relative weakness of certain 

 portions of them, and thus explain their tendency to 

 become varicose ? 



The external coat is. similar in every particular to External coat 



