364 



BLOOD-VESSELS. 



another by elastic fibres passing across the muscular bundles. In most arteries this 

 elastic tissue of the middle coat is but slightly developed, but in the aorta (fig. 424) 

 and carotid arteries and in some of the branches of the latter, it attains a consider- 

 able development, and since in them also elastic fibres are seen extending into the 

 subepithelial layer of the inner coat, the distinction between the inner and middle 

 coats as shown in section is far less marked than it is in ordinary arteries. There is 

 also a not inconsiderable amount of connective tissue in the middle coat of the 

 aorta. 



The muscular fibre-cells of the middle coat of the arteries (fig. 422 and fig. 423) 

 are seldom more than from ^ to ^ of an inch long and frequently, especially in 



Fig. 422. MUSCULAR FIBRE-CELLS FROM HUMAN ARTERIES. MAGNIFIED 350 DIAMETERS. 



1. From the popliteal artery ; a, natural ; b, treated with acetic acid. 2. From a small branch of 

 the posterior tibial (from Kb'lliker). 



Fig. 423. MUSCULAR FIBRE-CELLS FROM SUPERIOR THYROID ARTERY (MAN). 340 DIAMETERS. 



(E. A. S.) 



those arteries in which the elastic tissue of the middle coat is most developed, 

 present a very irregular shape with jagged extremities (fig. 423). Their nuclei are 

 distinctly rod-shaped and are often slightly curved. 



Bundles of white connective-tissue fibrils may also occur in small quantity in the 

 middle coat, the proportion increasing with the size of the artery. It is important 

 to note that the muscular tissue of 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 contractility of 

 the arteries, which depends on the muscular tissue of the middle coat, is little 

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

 branches. 



External coat (Tunica adventitia) (fig. 417, 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 figure 

 the elastic fibres are seen cut across). The elastic tissue is much more abundant 

 towards the inner part, next the muscular coat, and is frequently described as con- 

 stituting 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 



