MINUTE ANATOMY OF THE ARTERIES. 



271 



closely packed. In the more peripherically situated vessels 

 the quantity of this intermediate substance diminishes, and 

 the muscle cells are in closer proximity with one another. In 

 the larger arterial trunks, as the aorta, pulmonary, subclavian, 



Fig. 46. 



Fig. 46. Small artery from the brain of Man. , tunica adventitia ; 

 a, a nucleus of the tunica adventitia ; b, muscle nucleus ; c, elastic in- 

 ternal tunic ; d, cell membrane formed of fusiform cells. 



and carotid arteries, the intermediate substance is not only so 

 abundant that the short and isolated muscle cells, and the 

 smaller groups of such cells, are separated from one another by 

 large intervening spaces, but the elastic tissue also attains its 

 greatest development in the muscular layers. Associated with 

 the fine and narrow-meshed elastic-fibre networks which traverse 



