272 



THE BLOODVESSELS, BY C. J. EBERTH. 



the fibrous, granular intermediate substance, are a series of 

 lamellae of tolerably even width, composed of elastic bands and 

 fenestrated membrane. These, arranged at nearly regular in- 

 tervals, constitute septa, dividing the muscular tunic into 

 numerous layers. The lamellae are connected by numerous 

 oblique anastomoses, and are also continuous with the fine 

 elastic fibres. They chiefly pursue a transverse direction. 



In man, at least, there is always a layer of circularly 

 arranged muscle cells, which, however, are strengthened by 

 oblique or longitudinal muscular fasciculi, that are sometimes 

 situated externally, sometimes internally, to the circular fibre 

 layer, and sometimes occupy both positions. 



Scattered longitudinally and obliquely disposed muscle cells 

 are found in the descending thoracic aorta between the trans- 

 verse muscular fibres. The large vessels that, on account of 

 their loose connections, are easily moved, like those of the 

 viscera of man and mammals, the arteria lienalis, renalis, um- 

 bilicalis, and dorsalis penis, are particularly characterised by 

 longitudinal muscular bundles. 



The longitudinal muscles of the arteries are chiefly situated 

 in the tunica adventitia, especially in its internal and middle 



Fig. 47. 



Fig. 47. Transverse section of the coats of the basilar artery, a, 

 endothelium ; 6, elastic internal membrane ; c, muscle cells ; d, tunica 

 adventitia. 



layers, where, however, they seldom form a continuous layer, but 

 are united into fasciculi of greater or less strength (arteria renalis, 

 lienalis, dorsalis penis). A well-developed longitudinal muscu- 

 lar layer invests the circular fibrous layer, which is also strongly 

 marked in the arteries of the mesovarium of Batrachia. The 



