BLOOD- VESSELS 



103 



Arteries possess three coats: the intima (internal), media (mid- 

 dle), and adventitia (external). 



Fig. 73 represents a medium - sized typical artery. The intima, 

 or internal coat, A, B, C, consists of a layer of flattened endo- 

 thelial cells, which rest upon fibrous connective tissue, with a few- 

 elastic fibers. These structures are surrounded by a layer of elastic 

 tissue, the elastic lamina or fenestrated membrane, which is the 

 external limit of the intima. It appears in a transverse section as 

 a wavy (from contraction of the media) shining line, and is an 

 important element, from its relation to certain abnormalities of the 

 blood-vessels. The media, D, consists of alternate layers of 



FIG. 73. 



PARTLY 



TRANSVERSE SECTION OF A MEDIUM-SIZED ARTERY. 

 DIAGRAMMATIC. 



A. The endothelial cells in profile. 



B. Elastic and connective tissue supporting the endothelinm. 



C. The internal elastic lamina or fenestrated membrane. A, B, and C constitute the INTIMA 

 of the artery. 



D. The MEDIA. It consists of muscular and elastic tissues in alternating layers. 



E. Points to one of the elastic layers. 



F. The ADVENTITIA. Loose connective tissue, with few elastic fibers. 



elastic and muscular tissue. The adventitia, F, is composed of 

 fibrous connective tissue, containing some elastic elements. 



As we approach the larger arteries, the muscular tissue <jjmin- 

 ishes in quantity and the elastic tissue is increased. On the other 

 hand, the elastic element diminishes with preponderance of muscle 

 as we approach the smaller arteries, until we meet the arterioles, 

 the walls of which are made almost exclusively of involuntary mus- 

 cular fibers, surrounding a layer of endothelial cells. 



The walls of the capillaries consist of a single layer of flat- 

 tened endothelial cells cemented by their edges. The union is not 



