Y4 VESSELS. AKTEKIES. VEINS. CAPILLARIES, ETC 



much better seen; they form polygons of variable 

 regularity of outline, in which the central nucleus is 

 pretty uniformly apparent (PI. XV. fig. IV. 4). It 

 is to be remarked that these muscular fibres are dis- 

 tributed with perfect regularity throughout the whole 

 thickness of the middle coat. 



External coat. The external coat is formed by a close interlace- 

 ment of connective and elastic fibres, resembling felt. 

 The farther from its outer surface the greater the 

 amount of elastic fibres, and they generally run 

 parallel with the axis of the vessel (PL XV. fig. V. 

 1, 2; fig. VI. 1,2). 



In reviewing the structure of the walls of arteries 

 it is obvious that elastic fibre forms the frame-work 

 of all their coats ; but also, that in each separate coat 

 it is associated with another distinct and character- 

 istic element : in the internal, this is epithelium ; in 

 the middle coat, muscular, and in the external, con- 

 nective, fibre. Just in proportion as we approach 

 the smaller terminal arterial branches, the elastic 

 fibres tend to disappear, especially in the middle coat, 

 which finally becomes entirely muscular (PL XIV. 

 fig. VII). 



In the last and smallest branches of the arterial 

 tree which we can examine, those, for example, which 

 measure from T \th to ~th of a line in diameter, we 

 still recognise the three coats, but each one of them 

 is constituted by only a single lamella of tissue, com- 

 prising but a solitary anatomical element. Thus, the 

 outer coat consists of a very thin layer of connective 

 fibres mingled with some plasmatic cells (PL XV. 

 fig. VII. 1). The middle coat shows very short fusi- 



