370 PARASITES OF ANIMALS 



The changes in the intima are the least constant. They 

 present all stages of progressive and retrogressive metamor- 

 phosis, from simple induration to ulceration and calcification. 

 In the walls of the verminous aneurism one not unfrequently 

 finds all the pathological changes exhibited by atheroma in man. 

 Calcification is a common form of the retrograde process, and, 

 in very rare cases, may pass on to the formation of true bone. 



7. In addition to the palisade worms, one almost constantly 

 finds a parietal thrombus contained in the aneurism. It covers 

 the inner wall either partially or completely, being in the latter 

 case perforated for arterial offshoots. This clot may occlude 

 the artery, and it is not unfrequently continued into the arterial 

 branches (peripherally) or into the aorta (centrally). Amongst 

 the various changes that the clot undergoes, organisation of its 

 outermost layer and softening are the most frequent. The 

 constant occurrence of this clot is due to the presence of the 

 worms, to the inflammation, ulcerative and regressive affection 

 of the intima, and to the dilatation of the arterial tube. 



8. The palisade worms are seldom absent from aneurisms of 

 the horse. Their not being present is merely an accidental 

 circumstance. On the average, nine palisade worms go to a 

 verminous aneurism, and eleven in the horse. The highest 

 number of worms found in one horse reached 121. Not unfre- 

 quently, also, palisade worms, or their coverings in the form of 

 larval skins, are found in the aneurismal walls. The immigra- 

 tion and emigration of the palisade worms out of the intestine 

 into the aneurism, and the reverse, take place probably, as a 

 rule, within the arterial circulation. The path of the worm 

 does not appear to be always the same, inasmuch as they 

 can also wander through the peritoneal cavity. The worms 

 found in the aneurismal walls are probably mostly only strayed 

 specimens. 



9. From a comparative pathologico-anatomical point of view, 

 the developmental history of the aneurysma verminosum proves 

 that a circumscribed endo-arteritis can determine the formation 

 of an aneurism. 



10. Like the worm-aneurism itself, atheroma of the abdo- 

 minal arteries arises from a circumscribed acute and subacute 

 endo-arteritis. The histological changes in the secondary 

 atheroma of horses are perfectly analogous to those of the 

 spontaneous atheroma of man. Idiopathic atheroma, as seen 

 in man, does not occur any more in the horse than in the other 



