VESSELS. AETEEIES. VEINS. CAPILLAEIES, ETC. 



vessels which they accompany, rather than for the 

 innervation of the vessels themselves. They seem to 

 terminate by free ends, and it is uncertain whether 

 or not they reach the internal coat. 



SECT. III. CAPILLAEIES. The capillary vessels, capillaries. 

 which are the media of communication between the 

 arteries and veins, are exceedingly simple in their 

 structure. They are tubules of structureless sub- 

 stance, studded with oval nuclei. The larger the size 

 of the capillaries, the thicker are their walls, and the 

 greater the number of nuclei (PI. XV. fig. VIII ; PL 

 XVI. fig. I. 1). The smallest of them have such 

 exceedingly thin walls that they are portrayed by 

 only a single line. The transition from arteries and 

 veins to capillaries takes place insensibly, and by the 

 successive disappearance of the several organized ele- 

 ments which constitute the three tunics of a vessel. 



SECT. IV. LYMPHATIC VESSELS. After what has 

 been said concerning the histology of arteries and 

 veins, a few words only will be required for the 

 description of the structure of lymphatics. 



Their internal membrane consists of a simple layer 

 of epithelium supported by a web of elastic fibres of 

 extreme delicacy ; sometimes it appears to be reduced 

 to epithelium alone. .>; 



Their middle coat is composed almost exclusively 

 of muscular fibres arranged transversely ; elastic 

 fibres are very few in number (PL XVII. fig. II. 2, 

 3). Finally, the external coat differs from that, of 

 arteries and veins, by containing a large amount of 

 longitudinal muscular fibre in its deepest portion 

 (fig. II. 5 ; fig. III. 5). Their valves contain also 



