72 VESSELS. AETEEIES. VEINS. CAPILLAEIES, ETC. 



and vessels of very small size, such as capillaries, 

 recent specimens are required. 



Arteries. SECT. I. AETEEIES. When a section, either trans- 

 verse or longitudinal, including the whole thickness 

 of the wall of an artery, is placed under the micro- 

 scope, we recognise three distinct strata overlying 

 each other, which correspond to the three coats of 

 which the vessel is composed (PL XIV. fig. IV.). 

 The first, which is the thinnest, and uniformly dark 

 throughout its whole thickness, represents the in- 

 ternal coat (Fig. IV. 1). The second stratum, which 

 is transparent, and much thicker than the first, is the 

 middle coat (Fig. IV. 2). And, finally, the third 

 stratum, at least as thick as the second, and darker 

 in its deeper than in its more superficial portion, cor- 

 responds to the external coat. By employing a 

 magnifying power of 300 to 400 diameters it is easy 

 to make out the nature, as well as the arrangement 

 of the elements which constitute each of these coats. 

 The following is a summary of their microscopical 

 internal coat analysis : the internal tunic is limited, on its free sur- 

 face, by a layer of simple epithelium, which, exa- 

 mined in situ, seems to consist of oval nuclei, im- 

 bedded in a structureless substance ; the walls of its 

 cells are not distinguishable in consequence of their 

 extreme paleness (PI. XV. fig. I.). But, by teasing 

 out this membrane by the aid of needles, some cells 

 may be detached, which are recognizable as fusiform 

 in shape, with a very prominent bulge opposite the 

 situation of their nuclei (PI. XV. fig. II.), and which, 

 in this respect, resemble certain cells of the spleen. 

 Beneath this epithelial layer, which is in contact with 



