STRUCTURE OF ARTERIES. 121 



by the partial collapse of the vessel. It forms an important guide to 

 the pathologist in enabling him to determine which coat of the 

 artery is diseased.] 



In middle-sized and large arteries a few non-striped muscular fibres 

 are disposed longitudinally between two elastic plates or laminae (K. 

 Bardeleben). Along with the circular muscular fibres of the middle 

 coat, they may act so as to narrow the artery, and they may also aid in 

 keeping the lumen of the vessel open and of uniform calibre. It 

 is not probable that when they act by themselves they dilate the 



(2.) The Tunica media, or middle coat, contains much non-striped 

 muscle (c), which in the smallest arteries consist of transversely disposed 

 non-striped muscular fibres lying between the endothelium and the 

 T. adventitia, while a finely granular tissue with few elastic fibres forms 

 the bond of union between them. As we proceed from the very 

 smallest to the small arteries, the number of muscular fibres becomes 

 so great as to form a well-marked fibrous ring of non-striped muscle, in 

 which there is comparatively little connective tissue. In the large 

 arteries the amount of connective tissue is considerably increased, and 

 between the layers of fine connective tissue numerous (as many as 

 50) thick, elastic fibrous or fenestrated laminae are concentrically 

 arranged. 



A few non-striped fibres lie scattered amongst these, and some of 

 them are arranged transversely, while a few have an oblique or longi- 

 tudinal direction. 



The first part of the aorta and pulmonary artery, and the retinal arteries are 

 devoid of muscle. The descending aorta, common iliac, and popliteal have longi- 

 tudinal fibres between the transverse ones. Longitudinal bundles lying inside the 

 media occur in the renal, splenic, and internal spermatic arteries. Longitudinal 

 bundles occur both on the outer and inner surfaces of the umbilical arteries, which 

 are very muscular. 



(3.) The Tunica adventitia, or outer coat, in the smallest arteries 

 consists of a structureless membrane with a few connective tissue 

 corpuscles attached to it ; in somewhat larger arteries there is a layer 

 of fine fibrous elastic tissue mixed with bundles of fibrillar connective 

 tissue (d). In arteries of middle size, and in the largest arteries the 

 chief mass consists of bundles of fibrillar connective tissue containing 

 connective tissue corpuscles. The bundles cross each other in a variety 

 of directions, and fat cells often lie between them. Next the media 

 there are numerous fibrous or fenestrated elastic lamellae. In medium 

 sized and small arteries the elastic tissue next the media takes the 

 form of an independent elastic membrane (Henle's external elastic 



