SECT. 214.] STRUCTURE OF ARTERIES. 485 



have scon vessels. The inferior vena cava of the ox is shown to 

 be provided with numerous vessels, as far as to its tunica intiraa. 

 — Nerves, passing from the sympathetic and the spinal nerves, can 

 be demonstrated with facility on many arteries, yet frequently 

 appear only to accompany them. Where they penetrate into the 

 tissue of the vessels they run only within the outer tunic, and in 

 favourable cases, divisions and free terminations of their fine tubes 

 can be perceived in animals (see my Micros. Ana t. ii., 1, pp. 532, 

 533). Many arteries are completely destitute of nerves, as the 

 majority of those in the substance of the brain and spinal cord, 

 those of the choroid, of the placenta, as well as many arteries of 

 muscles, glands and membranes, and hence it is evident that the 

 walls of the arteries are not in such essential need of nerves, as is 

 usually supposed. This is still more evident in the case of veins, 

 for only on some of the larger ones have any nerves been demon- 

 strated : they have been observed on the sinuses of the dura mater, 

 on the veins of the vertebral canal, on the venae cavae, on the 

 common jugular, iliac, and crural veins, and on the hepatic veins. 

 Here also they arise from the sympathetic and the spinal nerves; 

 but they have not yet been investigated with reference to their 

 terminations. According to Luschka, they extend as far as to the 

 innermost coat ; but I have not yet succeeded in observing this. 



§ 214. For the sake of easier description, the arteries may be 

 divided into small, middle-sized, and large, according as their 

 middle coat is purely muscular, composed of muscular fibres and 

 elastic fibres intermixed, or of elastic fibres principally. This 

 division is the more natural, as, along with the alterations in the 

 structure of the middle coat, variations in several respects also 

 occur in the outer and inner coat. It is a distinguishing mark of 

 the arteries, that their middle coat possesses an unusual thick- 

 ness, and consists of numerous, regularly disposed laminae, whose 

 elements run in a transverse direction. In the largest arteries, 

 the tunica media is yellow, very elastic, and of great thickness; 

 as the arteries become smaller, it progressively decreases in thick- 

 ness and becomes more reddish and contractile, till, at last, im- 

 mediately before the capillaries, it appears very thin, and then 

 disappears. The whitish tunica intima is always much thinner 

 than the middle coat, and varies within narrower limits, although 

 it also bears a proportion to the size of the vessels ; whilst, on the 

 other hand, the tunica adventitia is actually considerably thinner 

 in the largest arteries than in those of middling calibre, where its 



