248 CIRCULATORY APPARATUS. 



ratlier long over it. I may be all the more brief in my remarks 

 concerning the two outer coats. The media, composed of smooth 

 muscular fibres, constitutes the main bulk of all the larger 

 ressels. The transverse, nucleated spindle-cells impart a highly 

 characteristic, annulose appearance to the smallest arteries and 

 veins ; in the larger vessels the muscular fibres are disposed in 

 bundles in a framework of fenestrated lamellae of elastic tissue. 

 From three to ten or more principal lamellae, separated by 

 regular intervals, lie j^arallel to the surface of the vessel ; each 

 lamella being connected with its neighbours on either side by 

 oblique buttresses. Tliese buttresses are also fenestrated, so that 

 the muscular bundles can pass uninterruptedly from one com- 

 partment to another. 



The adventitia consists mainly of unformed connective tissue 

 abundantly permeated by vessels and elastic elements (mem- 

 branes and fibres). In the larger veins, it may sometimes 

 contain longitudinal bundles of smooth muscular fibre. 



a. Inflamsiation. 

 a. Acute Inflanimation. 



§ 212. The phenomena of acute inflammation of the vessels 

 are intimately connected with those of coagulation in their in- 

 terior; for they are exclusively, or almost exclusively confined 

 to parts where a softening blood-clot is in immediate contact 

 with the inner wall of a vein or an artery, irritating the coats 

 of the vessel by the diffusible products of its disintegration. 

 This coincidence is so frequent, that it formerly led Dupiiytreyi 

 to invert the real order of sequence — to regard the acute in- 

 flammation of the vascular wall as the antecedent, and the 

 coagulation of the blood as the consequent. 



Even the most weighty and thorough changes operated by 

 acute inflammation are by no means striking to the naked eye. 

 We notice a hyperasmic state of the vasa vasorum, particularly 

 at the junction of the media with the adventitia, and a thicken- 

 ing of the vascular coats to three or four times their normal 

 diameter, so that an inflamed vein may not be distinguishable 

 on transverse section from an ordinary artery. The normal 

 smoothness of the inner surface gives place to a cloudy, opaque, 

 or even velvety aspect. In rare cases, we notice little col lee- 



