318 



THE CIRCULATION OF THE BLOOD. [CHAP. XXVin. 



separate readily when pulled in the transverse direction. They form 

 Fig. 186. a series of concentric layers, in 



number proportioned to the 

 thicknessof the artery, composed 

 of coarse yellow fibres which 

 branch and interlace freely. 



Upon fine transverse sections 

 of the middle coat of one of the 

 large arteries of man, or of the 

 ox, we may observe the peculiar 

 arrangement of these branching 

 fibres which give rise to the 

 tendency of this coat to split into 



Certain large fibres or rods of yellow elastic tissue are 

 disposed in successive concentric circles which pass transversely, and 

 sometimes obliquely round the artery. (Figs. 187, 189, 190). These 

 branch in a penniform manner (hence we propose to call them the 

 penniform fibres), and interlace with those on the same plane as 



Fig. 187. 



A portion of the circular fibrous tunic of the aorta of 

 the horse, to shov the reticulation formed by the in- 

 terlacement of its fibres. Magnified 200 diameters. 



lamellae. 



Section of the aorta of the ox, showing the arrangement of the two layers of the longitudinal 

 fibrous tunic and of the circular fibrous tunic. Magnified 250 diameters, a. The epithelial 

 tunic. 6. The internal layer of the longitudinal fibrous tunic, c. The external coarse layer of 

 the same. d. A small portion of the circular fibrous tunic : most of the fibres are cut across, 

 but a few which take an oblique course are seen in their whole length, and their pennifonn 

 branching is slightly indicated. 



well as with those on an inner and outer plane. The branches again 

 subdivide, and form, by their frequent anastomoses, that intricate 



