152 



MANUAL OF HISTOLOGY. 



middle being represented by the muscle- cells, and the first or 

 internal by the endothelial tube. The latter now receives the 

 name of intima. When the layers of its walls are arranged in 

 this simple manner the vessel is called an arteriole, and this 

 constitutes the type of all arteries. 



Arterioles, however, commonly contain a few additional 

 fibres between the intima and the media, as the first indication 

 of what afterward becomes a special layer. This structure, 

 known as the internal elastic coat, attains considerable devel- 

 opment in the larger vessels. With the growth of an artery in 

 calibre its individual coats are reinforced by additional layers. 

 Hence the thickness of the entire wall increases at the same 



Fio. 64. Fio. 65. Fia. 66. 



Fio. 6-i. Minute artery showing optical section of alternate groups of muscle-cells, and an external nu- 

 cleated membrane, representing the tunica a Ivcntitia. 



Fio. (55. A. intima ; B, delicate internal elastic coat ; E, media (as in Fig. 04) ;,D, adventitia. Artcriole, 

 from a child's mesentery. 

 , Fio. 66. Elastic internal tunic of the basilar arteries. 



time that its' structure is rendered more complex. But new 

 tissues never appear. Moreover, the increased thickness is not 

 uniformly proportionate to the enlarged calibre ; neither does it 

 take place by equal participation of the different tissues men- 

 tioned. In vessels of small and medium size there is a prepon- 

 derance of muscular over elastic elements. In the larger trunks 

 the reverse condition obtains. It is, therefore, proper to dis- 

 tinguish arteries of the muscular from those of an elastic type. 

 The latter class is represented by the principal distributing 

 trunks, all the remaining arteries belonging to the muscular 

 type. There exist, however, no abrupt lines of demarcation 

 between these main forms the one merging gradually into the 

 other. 



The interposition of the internal elastic coat between the 



