196 STRUCTURE OF ARTERIES. [BOOK i. 



pressure ; when they relax, they allow the internal pressure which 

 may exist, to distend the vessel and temporarily to increase the 

 calibre. 



When such a small artery breaks up into capillaries the 

 muscular fibres and elastic membrane disappear, the remnant of 

 the muscular coat being sometimes continued for a short distance 

 in the form of a single fibre straggling in a spiral fashion round 

 the artery towards the capillary ; all that is left is the epithelioid 

 lining of the inner coat with a little connective tissue to represent 

 the outer coat. 



109. The larger arteries resemble the minute arteries in so 

 far that their walls may be considered as composed of three coats, 

 but each of these coats is of a more or less complex nature, and 

 the minor details of their structure differ in different arteries. 



In such an artery as the carotid or radial, the three coats have 

 the following general characters. 



The inner coat is composed of a lining of epithelioid cells 

 resting not on a single delicate basement membrane, but on an 

 elastic layer of some thickness, consisting chiefly of a so-called 

 ' fenestrated ' elastic membrane or of more than one such mem- 

 brane, together with some amount of fine elastic fibres and in 

 some cases at all events a small quantity of white connective 

 tissue. A 'fenestrated' membrane is a membrane composed of 

 the same substance as the elastic fibres, perforated irregularly 

 with holes, and more or less marked with indications of fibres; 

 it may be regarded as a feltwork of elastic fibres, fused or 

 beaten out, as it were, in a more or less complete membrane, 

 some of the meshes of the feltwork remaining as 'feriestras' 

 and traces of the fibres being still left. Such fenestrated 

 membranes, some thick, some thin, occur both in the inner 

 and middle coats of the larger arteries ; and in the inner coat, 

 usually immediately under the epithelioid lining, there is in most 

 large arteries a conspicuous membrane of this kind, sometimes so 

 thick as to give a very distinct double outline in sections of the 

 artery even under moderate powers. Beneath this there may be 

 other similar fenestrated membranes, or a feltwork of fine elastic 

 fibres held together by a very small quantity of white connective 

 tissue. In the aorta, and in some other arteries, the epithelioid 

 cells rest immediately not on an elastic membrane but on a thin 

 layer of so-called ' sub-epithelioid ' tissue, which consists of con- 

 nective tissue corpuscles imbedded in a homogeneous or very 

 faintly fibrillated matrix or ground substance. 



The epithelioid cells are disposed longitudinally, that is, with 

 their long diameters parallel to the axis of the artery, and a 

 similar longitudinal arrangement obtains to a greater or less 

 extent in the underlying elastic elements. When after death 

 the arteries, emptied of blood, become narrowed or constricted 

 by the contraction of the muscular elements of the middle coat, 



