SOLIDUNGULA 369 



siderable hypertrophy of the muscular layer, by the tough fibrous 

 capsule formed in many cases by the connective tissue of the 

 mesentery, and by the adhesion of the intestines to the perpen- 

 dicular and free-lying anterior mesenteric artery ; in particular 

 this last-named circumstance does not allow of any very consider- 

 able shortening of the mesenteric artery, which would necessarily 

 be accompanied by considerable dilatation of the arterial tube. 



4. The favorite seat of the worm-aneurism is the trunk of the 

 anterior mesenteric artery, directly at its origin from the abdo- 

 minal aorta. Most frequently that part of the arterial trunk is 

 dilated from which the arteria ilea, csecales, and colica inferior 

 (arteria ileo-cceco-colica) arise, less frequently the arteria colica 

 superior at its origin, and the arteries of the caecum and colon 

 in their course in the meso-caecum and meso-colon. The 

 verminous aneurism also occurs in the coeliac artery (Bauch- 

 schlagader), in the posterior mesenteric artery (Gekros-arterie), 

 in the renal artery, and in the abdominal aorta. A horse is not 

 unfrequently afflicted with several aneurisms of this kind at one 

 and the same time. Thus in one case (described by Bellinger) 

 there were six of these aneurisms affecting the abdominal aorta 

 and its branches in the same horse. The verminous aneurism 

 may occur from the sixth month of life onwards, and with 

 increasing age ; the number of horses free from such aneurisms 

 becomes continually smaller. 



5. The size of the aneurism varies between that of a pea and 

 that of a man's head. The dilatation is, as a rule, equal on all 

 sides, the form being usually thumb-shaped or bottle-shaped, 

 passing into that of a cone or long oval figure. This general 

 configuration is principally due to the free and moveable situa- 

 tion of the anterior mesenteric artery. 



6. In contrast to aneurisms in man, the walls of the worm- 

 aneurism of the horse are almost without exception indurated. 

 In addition to the mesenteric connective tissue, all the arterial 

 coats, and especially the tunica media, generally take part in this 

 induration. The hypertrophy of the media, which stands 

 unique in respect of what is known of arterial disease, forms a 

 compensatory action of the arterial wall, analogous to the 

 muscular hypertrophy of the heart in valvular disease. This 

 change in the media points to the fact that in the development 

 of aneurism in man the early disturbance of the nutritive pro- 

 cess in the tunica media is not a less essential factor than the 

 degeneration of the tunica intima. 



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