1126 



GENERAL ANATOMY OR HISTOLOGY. 



THE VASCULAR SYSTEM. 



The Vascular System, exclusive of its central organ, the heart, is divided into 

 four classes of vessels : the arteries, capillaries, veins, and lymphatics ; the minute 

 structure of these vessels will be briefly described here, the reader being referred 

 to the body of the work for the details of their ordinary anatomy. 



Structure of Arteries (Fig. 665). The arteries are composed of three coats: 

 internal or endothelial coat (tunica intima of KSlliker) ; middle muscular coat 



(tunica media] ; and external connective- 

 tissue coat (tunica adventitia]. 



The two inner coats together are very 

 easily separated from the external, as by 

 the ordinary operation of tying a ligature 

 on an artery. If a fine string be tied for- 

 cibly upon an artery and then taken off, 

 the external coat will be found undivided, 

 but the internal coats are divided in the 

 track of the ligature and can easily be fur- 

 ther dissected from the outer coat. The 

 inner coat can be separated from the middle 

 by a little maceration, or it may be stripped 

 off in small pieces; but, on account of its 

 friability, it cannot be separated as a com- 

 plete membrane. It is a fine, transparent, 

 colorless structure which is highly elastic, 

 and is commonly corrugated into longitudi- 

 nal wrinkles. The inner coat consists of 



1. A layer of pavement-endothelium, the cells 

 of which are polygonal, oval, or fusiform, 

 and have very distinct round or oval nuclei. 

 This endothelium is brought into view most 

 distinctly by staining with nitrate of silver. 



2. A subendothelial layer, consisting of 

 delicate connective tissue with branched 

 cells lying in the interspaces of the tissue. 

 In arteries of less than a line in diameter 

 the subendothelial layer consists of a single 

 stratum of stellate cells, and the connective 

 tissue is only largely developed in vessels 

 of a considerable size. 3. An elastic or 

 fenestrated layer, which consists of a mem- 

 brane containing a network of elastic fibres, having principally a longitudinal 

 direction and in which, under the microscope, small, elongated apertures or per- 

 forations may be seen, giving it a fenestrated appearance. It was therefore 

 called by Henle the fenestrated membrane. This membrane forms the chief 

 thickness of the inner coat, and can be separated into several layers, some of 

 which present the appearance of a network of longitudinal elastic fibres, and 

 others present a more membranous character, marked by pale lines having a 

 longitudinal direction. The fenestrated membrane in microscopic arteries is a 

 very thin layer, but in the larger arteries, and especially in the aorta, it has a 

 very considerable thickness. 



The middle coat (tunica media) is distinguished from the inner by its color and 

 by the transverse arrangement of its fibres, in contradistinction to the longitudinal 

 direction of those of the inner coat. In the smaller arteries it consists principally 

 of muscular tissue, being made up of plain muscle-fibres in fine bundles, arranged 

 in lamella: and disposed circularly around the vessel. These lamellae vary in 

 number according to the size of the vessel ; the very small arteries having only a 



FIG. 665. Transverse section through a small 

 artery and vein of the mucous membrane of the 

 epiglottis of a child. Magnified about 350 diame- 

 ters. (Klein and Noble Smith.) A. Artery, show- 

 ing the nucleated endothelium, e, which lines it: 

 the vessel being contracted, the endothelial cells 

 appear very thick. Underneath the endothelium 

 is the wavy elastic intima. The chief part of the 

 wall of the vessel is occupied by the circular mus- 

 cle-coat m ; the staff-shaped nuclei of the muscle- 

 cells are well seen. Outside this is a, part of the 

 adventitia. This is composed of bundles of con- 

 nective-tissue fibres, shown in section, with the 

 nuclei of the connective-tissue corpuscles. 

 The adventitia gradually merges into the sur- 

 rounding connective tissue, v. Vein showing 

 a thin endothelial membrane, e, raised acciden- 

 tally from the intima, which on account of its 

 delicacy is seen as a mere line on the media m. 

 This latter is composed of a few circular un- 

 striped muscle-cells, a. The adventitia, similar 

 in structure to that of an artery. 



