STRUCTURE OF ARTERIES. 399 



branches to form the basilar ; by inosculation, where two vessels 

 of nearly equal size become continuous and form an arch, from 

 which may spring smaller vessels, as in the arteries of the 

 intestines ; by transverse branches, where two collateral arteries 

 are joined by a small communicating branch, as between the 

 umbilicals as they approach the placenta. In the smaller 

 arteries anastomoses are frequent, forming a network which 

 pervades almost every tissue of the body. This is a point of 

 great importance, since the circulation can be thus carried on 

 after the main artery of the region has been obliterated, blood 

 being conveyed from neighbouring sources by the anastomosing 

 arteries, and it is proved that under such circumstances the 

 smaller arteries of the part increase in size. 



STRUCTURE OF ARTERIES. 



Arteries are dense and elastic, possessing great power of resist- 

 ance. When empty, they are of a pale buff colour, and preserve 

 their cylindrical form. Their walls consist of three tunics au 

 internal, middle, and external. The internal, serous, or tunica 

 intima. is the thinnest, and continuous with the lining membrane 



O 



of the heart. It is made up of two layers ; an inner, consisting 

 of a layer of endothelial scales ; and an outer, transparent, whitish, 

 highly elastic, and perforated, being known as the fenestrated 

 membrane of Henle. When the artery is empty, it is thrown 

 into folds, longitudinal in the large, transverse in the small 

 arteries ; and when distended, it is smooth and glistening. The 

 tunica media, or contractile middle coat, is elastic, dense, and 

 of a yellow colour, consisting of non-striated muscular, and elastic 

 fibres ; thickest in the larger arteries, it becomes thinner in 

 the smaller. In the largest arteries this coat is composed chiefly 

 of elastic tissue ; in the smallest vessels it is almost entirely 

 muscular. The tunica extima, or external coat, is made up of 

 areolar tissue and elastic fibres, very thin in the large, as thick 

 as the middle coat in the smaller, while in the smallest arteries 

 it is rudimentary or wanting. The external surface is loose, and 

 connected by laminae with the sheath. 



The arterial sheaths consist of cellular tissue, intimately con- 

 nected with the surrounding textures. The artery has only a 

 loose attachment to the sheath, so as to allow of free glidincr on 

 its inner surface. An artery is usually accompanied by a vein, 



