604 



THE ABTEBIES. 



these vessels to remain open when they are emptied of blood. The ancients 

 believed this was their normal condition, and that they were filled with air during 

 life. This was a grave error, as they contain nothing but blood. The gaping of 

 the arteries must be attributed solely to the physical properties of the tissues 

 composing their walls. 



These walls comprise three tunics — an internal, middle, and external. 



The internal tunic {tunica intima) is continuous with the endocardium of the 

 left heart on the one part, and on the other with the capillaries and veins. For 

 a long time it has been regarded as a serous membrane, but it has not absolutely 

 the same texture. It is composed of a simple endothelial layer which is in contact 

 with the blood, and is formed by fusiform cells that slightly bulge where their 

 nucleus is. The endothelium lies upon a layer of very fine elastic tissue (the 

 fenestrated membrane of Henle), perforated by openings which are occupied by 



Fig. 361. 



EPITHELIAL CELLS OP 

 BLOOD-VESSELS. 



a, b, From a vein ; c, 

 from an artery. 

 Magnified 350 dia- 

 meters. 



FENESTRATED MEMBRANE 

 FROM. THE CAROTID 

 ARTERY OF THE HORSE. 



Magnified 350 diame- 

 ters. 



NETWORK OF COARSE 

 ELASTIC TISSUE FROM 

 MIDDLE COAT OF PUL- 

 MONARY ARTERY OF 

 THE HORSE, THE FIBRES 

 BEING PIERCED WITH 

 CIRCULAR OPENINGS. 



Magnified 350 diame- 

 ters. 



a slightly fibrillated connective substance, and by ramifying and anastomosing 

 cells. Beneath this layer there is another, also composed of fine elastic and 

 connective fibres lying in a transverse direction, and which is attached to an 

 elastic layer {internal elastic layer) that determines the external limit of the deep 

 tunic of arteries in the aorta ; this layer is separated from the preceding by the 

 network of Langhans. 



The middle tunic {tunica media) is remarkable for its thickness, its elasticity, 

 and its yellow colour in the principal vessels. It has for base several elastic parallel 

 layers, united by means of networks of fibres of the same kind, and having spaces 

 between them which are occupied by connective tissue and non-striped muscular 

 fibres ; these are arranged in a circular manner aroui.d the vessels. In the umbilical 

 and splenic (and also in the aoita, iliac, anterior mesenteric, and renal) arteries, 

 there are longitudinal muscular fibres. The proportion of these two elements 

 varies with the size and situation of the artery. In the large trunks — such aa 

 the aorta, the elastic is more abundant than the contractile tissue ; in the 

 medium-sized vessels they are about equal ; but in the small arteries, in which 

 the contractile force of the heart is lost because of their distance from it, the 

 muscular fibres almost exclusively compose the middle tunic. 



