170 BLOOD-VESSELS. 



small quantity, the proportion of wliich increases with the size of the 

 artery. It is important further to note that the muscular tissue oi the 

 middle coat is more pure in the smaller arteries, and that the admixture 

 of other tissues increases in the larger-sized vessels ; in these, moreover, 

 the muscular cells are smaller. Accordingly, the vital contractility of 

 the arteries, which depends on their middle coat, is very little marked 

 in those of large size, but becomes much more conspicuous in the 

 smaller branches. 



External coat. {Tunica advenUtia of the German writers) (fig. 107, d). 

 This is composed mainly of fine and closely- felted bundles of white 

 connective tissue, together with a variable amount of longitudinally 

 disposed elastic tissue between the bundles (in the fig. this is seen cut 

 across). This is much more abundant towards the inner part, next 

 the muscular coat, and is frequently descr ibed as constituting here a 

 distinct elastic layer : it is most marked in arteries of medium calibre, 

 becoming thinner, and at length gradually disappearing in those of 

 small size. 



In large and middle-sized arteries the bundles of white connective 

 tissue chiefly run diagonally or obliquely round the vessel, and their 

 interlacement becomes much more open and lax towards the surface of 

 the artery, where they connect the vessel with its sheath or with other 

 surrounding parts. Longitudinally arranged contractile fibre-cells have 

 been described by various observers in the external coat of some of the 

 larger arteries, and they are said to be occasionally present amongst 

 the circularly disposed fibres of the middle coat, and even in the sub- 

 epithelial layer of the internal. The white tissue is usually of great 

 proportionate thickness in the smaller arteries. - 



Some arteries have much thinner coats than the rest, in proportion 

 to their calibre. This is strikingly the case with those contained within 

 the cavity of the cranium, and in the vertebral canal ; the difference 

 depends on the external and middle coats, which in the vessels referred 

 to are thinner than elsewhere. 



Vessels and Nerves of Arteries. — The coats of arteries receive 

 small vessels, both arterial and venous, named vasa vasorum, which 

 serve for their nutrition. The little nutrient arteries do not pass imme- 

 diately from the cavity of the main vessel into its coats, but are derived 

 from branches which arise from the artery (or sometimes from a neigh- 

 bouring artery), at some distance from the point where they are 

 ultimately distributed, and divide into smaller branches within the 

 sheath, and upon the surface of the vessel, before entering its coats. 

 They form a network in the tissue of the external coat, from which a 

 few penetrate into the middle coat, and follow the circular course 

 of its fibres; none have been discovered in the internal coat, unless 

 the observations of Jiische and Arnold are to be trusted, who affirm 

 that they have seen vessels in that situation. Minute venules return 

 the blood from these nutrient arteries, which, however, they do not 

 closely accompany, and discharge it into the vein or pair of veins which 

 usually run alongside the artery. Lymphatics are present in the outer coat. 



Arteries are generally accompanied by larger or smaller nerves ; and 

 when, in the operation of tying an artery, these happen to be included 

 along with it in the ligature, great pain is experienced, but the vessel 

 itself, when in a healthy condition, is insensible. Nerves are, neverthe- 

 less, distributed to the coats of arteries, probably for governing their 



