THE VASCULAR SYSTEM. 



1127 



single layer, and those not larger than one-tenth of a line in diameter three or four 

 layers. It is to this coat that the great thickness of the walls of the artery is 



Fio. 666. Longitudinal section of artery and vein, a, an artery from the mesentery of a child, .062'", and 6, 

 vein .067'" in diameter, treated with acetic acid and magnified 350 times, a. Tunica adventitia. with elongated 

 nuclei. /3. Nuclei of the contractile fibre-cells of the tunica media, seen partly from the surface, partly appar- 

 ent in transverse section. . y. Nuclei of the endothelial cells. 5. Elastic longitudinal fibrous coat. 



mainly due (Fig. 665, A, m). In the larger vessels, as the iliac, femoral, and 

 carotid, elastic fibres unite to form lamellae, which alternate with the layers of 



Endothelial and sub- 

 endothelial layer of 

 ' inner coat. 

 "Elastic layer. 



Innermost layers of 

 middle coat. 



V ^T 



* ^ - 



^Cp^ Outermost 'Layers of 



middle coat. 



._ Innermost part 

 outer coat. 



Outer part of outer 

 coat. 



FIG. 667. Section of 



artery. (After Grunstein.) 



muscular fibre and are united by elastic fibres which pass between the muscular 

 bundles, and are connected with the fenestrated membrane of the inner coat (Fig. 

 667). In the largest arteries, as the aorta and innominate, the amount of elastic 



