868 



THE VASCULAE SYSTEM. 



T. media 

 T. intima 



C B A2 A* 



FIG. 747. STRUCTURE OF BLOOD-VESSELS (diagrammatic). 



U, Capillary with simple endothelial walls. A2, Larger capillary 

 with connective tissue sheath, "adventitia capillaris." B, 

 Capillary arteriole showing muscle cells of middle coat, few 

 and scattered. C, Artery muscular 

 media forming a continuous layer. 



in different tissues. The small arteries which end in them are known as capillary 



arterioles, and the venous radicles which commence from them are appropriately 



termed capillary veins. 



Structure of Arteries and Veins. The delicate elastic endothelial membrane 



which forms the wall of the simplest capillaries extends also, as a continuous lining, 



throughout the whole of the blood-vascular system. In the arteries the con- 

 stituent cells are fusiform, narrow, and pointed, whilst in the veins they are some- 

 what shorter and broader. 



The most essential structural difference between capillaries on the one hand and 



the arteries and veins which they unite together on the other, is the presence, in 



both the arteries and the veins, of 



T. externa/^ . ,...,.... . ^^r^^^z^\\ in voluntary muscular fibres which 



are interposed between the endo- 

 thelial lining and the outer con- 

 nective tissue sheath. In small 

 vessels, e.g. capillary arterioles, the 

 muscle cells are few in number and 

 more or less scattered. In larger 

 vessels the walls are stronger and 

 ments of the tunica thicker, muscular fibres are more 

 numerous and form a continuous 



layer, whilst yellow elastic and ordinary white connective tissue are present in 



varying proportions. The walls of the larger vessels are, therefore, complex, and 



numerous strata may be distinguished ; which, for convenience, are regarded as 



forming three layers, known as the tunica intima and the middle and outer tunics. 



Superadded to the tunics is the investing fibrous sheath or vagina vasis. 



Structure of Arteries. The walls of arteries are stronger and thicker than 



those of veins of corresponding size, the tunica intima and middle tunic being 



particularly rich in elastic and muscular elements. 



Tunica Intima. In the tunica intima the endothelial lining is strengthened 



by the addition of yellow elastic tissue, the fibres 



of which are arranged in such a manner as to 



simulate a fenestrated membrane. In arteries of 



medium size the elastic lamina is separated from 



the endotheliunl by a layer of connective tissue 



consisting of branched cells and numerous fibrils. 



In the larger arteries the subendothelial connective 



tissue is considerably increased, and delicate elastic 



fibres appear which connect it with the more ex- 

 ternally situated and fenestrated elastic layer. 



Tunica Media. In the capillary arterioles the 



tunica media consists solely of scattered unstriped 



muscle fibres ; the individual fibres are circularly dis- 

 posed, but do not entirely surround the vessel. In 



small arteries the muscle cells are so much increased 



in number that they form a continuous though 



thin layer. The larger arteries have two or more 



layers of muscle cells, and the greater thickness of 



their walls is mainly due to the increase of the 



muscular elements of the middle coat. In the 



larger vessels delicate laminae of elastic tissue 



alternate with the layers of muscular fibres, and 



in the aorta and the carotid arteries, as well as 



in some of the branches of the latter, the elastic 



elements largely preponderate. In the first part 



of the aorta, in the pulmonary artery, and in the 



arteries of the retina, the muscular fibres are entirely replaced by elastic tissue. 

 Tunica Bxterna. The outer tunic of an artery consists almost entirely c 



fibrillated connective tissue, in which lie many connective tissue corpuscles 



FIG. 748. TRANSVERSE SECTIOI 

 THROUGH THE WALL OF A L.ARG) 

 ARTERY. 



A, Tunica intima. B, Tunica media. 

 C, Tunica externa. 



