144 



THE CIRCULATION. 



Fig. 40.' 



distension from the force of the heart's action. (2.) It 

 serves another purpose also in affording a suitable tissue 

 for the ramifications of the vasa vasorum, or nutritive vessels 

 for the supply of the arterial walls. 



The internal arterial coat is formed by layers of elastic 

 tissue, the outer portion consisting of coarse longitudinal 

 branching fibres, and the inner of a very thin and brit- 

 tle membrane which possesses ' little elasticity, and is 

 thrown into folds or wrinkles when the artery contracts. 

 This latter membrane, the striated or fenestrated coat 

 of Henle, is peculiar in its tendency to curl up when 

 peeled off in thin films from the artery, and in the 

 perforated and streaked appearance which it presents 



under the microscope. Its 

 inner surface is lined with a 

 delicate layer of epithelium, 

 composed of thin squamous 

 elongated cells, which make 

 it smooth and polished, and 

 furnish a nearly impermea- 

 ble surface, along which the 

 blood may flow with the 

 smallest possible amount of 

 resistance from friction. 



The middle coat is the seat 

 of those properties by which 

 arteries chiefly influence the circulation. The outer portion 

 of this coat is made up, chiefly, of fibres of yellow elastic 

 tissue, disposed for the most part circularly, and con- 

 stituting, as Hunter named it, the elastic coat. The inner 

 consists also of circular fibres of yellow elastic with a 

 sparing amount of white fibrous tissue, but mingled with 

 these, sometimes in alternate layers, and having the same 

 transverse direction, are pale, flat fibres, or " fibre-cells" 



* Fig. 40. Portion of fenestrated membrane from the crural artery, 

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



