THE ESSENTIALS OF HISTOLOGY 



layer in the form either of elastic networks or of a fenestrated mem- 

 brane. In some arteries there is a layer of fine connective tissue in- 

 tervening between the epithelium and the fenestrated membrane (sub- 

 epithelial layer). 



FIG. 109. TRANSVERSE SECTION OF PART OF THE WALL OF THE POSTERIOR TIBIAL 

 ARTEKY. (75 diameters.) 



a, epithelial and sub epithelial layers of inner coat ; 6, elastic layer (fenestrated membrane ) of 

 inner coat, appearing as a bright line in section ; c, muscular layer (middle coat) ; d, outer 

 coat, consisting of connective-tissue bundles. In the interstices of the bundles are some 

 connective-tissue nuclei, and, especially near the muscular coat, a number of elastic fibres 

 cut across. 



FIG. 110. EPITHELIAL, LAYER LINING 



THE POSTERIOR TIBIAL ARTERY. 



(250 diameters.) 



FIG. 111. PORTION OF FENESTRA- 

 TED MEMBRANE FROM AN ARTERY. 

 (Toldt.) 



(i, 6, c, perforations. 



The middle coat consists mainly of circularly disposed plain mus- 

 cular fibres, but it is also pervaded in most arteries by a network of 

 elastic fibres which are connected with the fenestrated membrane of 

 the inner coat and are sometimes almost as much developed as the 

 muscular tissue itself. This is especially the case with the larger 

 arteries such as the carotid and its immediate branches, but in the 

 smaller arteries of the limbs the middle coat is almost purely composed 

 of muscular tissue. 



