CHAP. XXVIII.] THE MIDDLE COAT OF ARTEKIES. 317 



artery suffers from so much ; in diseased or injured states of the 

 other coats, it preserves the integrity of the tube, and prevents any 

 serious interruptions to the circulation; the wall of many aneu- 

 risms consists in great part of this tunic : and on the application 

 of a ligature, while the inner and middle coats give way under the 

 pressure, this tunic resists and preserves its continuity for a time. 



Of the Middle, or Fibrous Coat. This tunic constitutes the 

 principal portion of the arterial wall. It is in greatest part com- 

 posed of yellow elastic fibrous tissue ; but it likewise contains some 

 white fibrous tissue, and also some of the unstriped muscular fibre. 



When a large artery, as the human aorta, or the aorta of a horse or 

 an ox, is cut either longitudinally or transversely, two very distinct 

 portions maybe observed on examining the surface of the section with 

 the naked eye. These are, an internal portion, quite yellow in colour, 

 and constituting not more than a tenth or a twelfth of the thickness of 

 the whole tunic; and an internal portion of a grayish-yellow colour. 



The internal portion, which we shall call the longitudinal fibrous 

 tunic, is composed of longitudinal fibres of yellow fibrous tissue, 

 disposed in two planes, Fig. is4. 



forming an internal and 

 an external layer. The 

 internal layer is in inti- 

 mate contact with the 

 epithelium, and consists 

 of fine pale somewhat flat 

 not branching fibres, im- Finely fibroug layer of the longitudinal fibrous tunic of the 

 bedded in a hyaline mem- aorta of the horse - Magnified 200 diameters. 

 brane which peels off readily in the length of the vessel, and when se- 

 parated from connexion with the adjacent layer assumes a coiled form, 

 as shown in fig. 184. These fibres are not altered in any degree by 

 the action of acetic acid. The external layer is composed of fibres of 

 elastic tissue, which also take a longitudinal direction, but are much 

 coarser, and branch freely, forming a Fig. iss. 



very intricate interlacement (fig.185) . 



The external grayish -yellow por- 

 tion of the fibrous coat of arteries 

 forms nine-tenths or eleven- twelfths of 

 the thickness of the wall of the artery, 



and may be distinguished from the Coarsely fibrous layer of the longittidmal 

 *^ .11 / fibrous tunic of the aorta of the horse. Mag- 



mternal portion by the name ot the nined 200 diameters. 



circular fibrous tunic. It consists entirely of transverse fibres, which 



surround the artery at right angles to its long axis. These fibres 



