CHAP, iv.] THE VASCULAR MECHANISM. 195 



In the first place, its walls being permeable are adapted for 

 carrying out that important interchange between the blood and 

 tissue, which, as we have more than once said, takes place almost 

 exclusively in the capillary regions. In the second place, the 

 extensibility and elasticity of its walls permit it to adapt its calibre 

 to the amount and force with which the blood is flowing into it. 



108. Arteries. The wall of a minute artery, i.e. of one 

 which is soon about to break up into capillaries, and which is 

 sometimes spoken of as an arteriole, consists of the following parts. 



The inside is lined with a layer of fusiform epithelioid cells, 

 very similar to those of a capillary and similarly cemented together 

 into a membrane. The long diameter of these fusiform cells, which 

 are sometimes very narrow, is placed parallel to the axis of the 

 artery. 



Outside this epithelioid lining comes a thin transparent 

 structureless or finely fibrillated membrane, seen in an optical 

 or other section of the artery as a mere line. This membrane, 

 which serves as a supporting membrane, basement membrane, or 

 membrana propria, for the epithelioid cells, is similar in chemical 

 nature and in properties to the elastic fibres found in connective 

 tissue, and hence is spoken of as the elastic membrane. The 

 epithelioid cells and the elastic membrane together are often 

 spoken of as forming the inner coat (tunica intima) of the artery. 



Wrapped transversely in a more or less distinctly spiral manner 

 round this inner coat, and imbedded in a small quantity of 

 connective tissue, lie a number of plain muscular fibres, arranged 

 in the smallest arteries in a single layer, in the larger but still 

 small arteries in more than one layer. This forms in these 

 arteries the middle or muscular coat (tunica media). Outside 

 this muscular coat comes the external coat (tunica extima), con- 

 sisting of connective tissue the bundles of which are disposed for 

 the most part longitudinally and contain a number of connective 

 tissue corpuscles and a relatively large number of elastic fibres. 

 This outer coat is continuous with the connective tissue bed in 

 which the artery lies. 



A minute artery then differs from a capillary, in the thickness 

 of its walls, whereby the permeability so characteristic of the 

 capillary is to a great extent lost, in the distinct development of 

 elastic elements, the elastic membrane of the inner coat, and the 

 elastic fibres of the outer coat, whereby elastic qualities are 

 definitely assured to the walls of the vessel, and lastly and chiefly 

 by the presence of distinct muscular elements. It is obvious, that 

 while by the development of elastic elements, passive changes of 

 calibre have a greater scope than in the capillary, active changes 

 in calibre, which in the capillary are at least obscure, are assured 

 to the artery by the muscular elements. When these transversely 

 disposed muscular fibres contract, they must narrow the calibre of 

 the artery, and may do that against even very considerable internal 



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