134 THE CIRCULATION. 



or wrinkles when the artery contracts. This latter mem- 

 brane, the striated or fenestrated coat of Henle, is pecu- 

 liar in its tendency to curl up, when peeled off from the 

 artery, and in the perforated and streaked appearance 

 Fig. 41.* which it presents under 



the microscope. Its inner 

 surface is lined with a de- 

 licate layer of epithelium, 

 composed of thin squamous 

 elongated cells, which make 

 it smooth and polished, and 

 furnish a nearly imperme- 

 able surface, along which 

 the blood may flow with the 

 smallest possible amount of 

 resistance from friction. 

 The walls of the arteries, with the possible exception of 

 the epithelial lining and the layers of the internal coat 

 immediately outside it, are not nourished by the blood 

 which they convey, but are, like other parts of the body, 

 supplied with little arteries, ending in capillaries and veins, 

 which, branching throughout the external coat, extend for 

 some distance into the middle, but do not reach the internal 

 coat. These nutrient vessels are called vasa vasorum> 

 Nerve-fibres are also supplied to the walls of the arteries. 



The function of the arteries is to convey blood from the 

 heart to all parts of the body, and each tissue which enters 

 into the construction of an artery has a special purpose to 

 serve in this distribution. 



(l.) The external coat forms a strong and tough invest- 

 ment, which, though capable of extension, appears princi- 

 pally designed to strengthen the arteries ^and to guard 

 against their excessive distension from the force of the 



* Fig. 41. Portion of fenestrated membrane from the crural artery, 

 magnified 200 diameters, a, b, c, perforations (from Henle). 



