676 



HUMAN ANATOMY. 



coat, which then becomes a purely muscular tunic and, before the capillary is reached, 

 is reduced to a single layer of muscle-cells. In the precapillary arterioles the muscle 

 no longer forms a continuous layer, but is represented by groups of fibre-cells that 



Fig. 636. 



Intima 



Internal 



elastic 



membrane 



Media 



External elastic 



membrane 



Adventitia 



Transverse section of artery of medium size, stained to show elastic tissue. X 100. 



partially wrap around tlfc vessel, and at last are replaced by isolated elements. After 

 the disappearance of the muscle-cells, the blood-vessel has become a true capillary. 

 The adventitia shares in the general reduction and gradually diminishes in thickness 

 until, in the smallest arteries, it consists of only a few fibro-elastic strands outside the 

 muscle-cells. 



In the large arteries, on the other hand, the intima and media chiefly undergo 

 augmentation. Although the inner coat greatly thickens and contains a large 

 amount of fibrous tissue and elastica, a conspiciaous internal elastic membrane, as 

 seen • in the smaller vessels, is lacking, since the elastic plates and membranes are 

 now so abundant that the local accumulation is no longer striking, the boundary 

 between the inner and middle 



coats being, therefore, less •^7- 



sharply defined. The character 

 of the thickened media also 

 changes, the muscular tissue be- 

 ing relatively reduced and over- 

 shadowed by the excessive de- 

 velopment of the fibro-elastic 

 tissue, which is arranged in reg- 

 ularly disposed lamellae separa- 

 ting the muscle-bundles and 

 conferring a more compact and 

 denser character to the wall of 

 the vessel. The adventitia, 

 while relatively thinner than in 

 arteries of medium size, is also 

 increased and consists of robust 

 fibres and plates of elastica, many of which are longitudinally disposed and irreg- 

 ular, although strong, bundles of fibrous tissue. E.xceptionally, longitudinal strands 

 of muscle appear in the outer coat next the media. In the roots of the aorta and 



Small arteries in which muscular coat is reduced to single 

 layer of cells. X 150. 



