Aneurism. 357 



aorta too, from its size and direction on leaving the heart, is in 

 the direct Hne of the strongest blood current, and under long con- 

 tinued, forced and violent efforts (as in racing, hunting, and 

 heavy uphill draughts), has to sustain an extraordinary blood 

 pressure. Bouley claims as an additional cause the pressure of a 

 loaded colon. This is also the point of all others where the ves- 

 sels suffer from the presence of the larval strongyli. From what- 

 ever cause originating, congestion of the arterial coats leads to 

 more or less attenuation, softening or lack of cohesion, and they 

 tend to yield under the blood pressure. Similar conditions op- 

 erate on the smaller vessels in different parts of the body, and 

 thus overstretching, contiguous inflammation, and excessive 

 blood pressure cause such lesions in the chest, trunk and limbs. 



Another cause is embolism which by blocking an arterj^ at once 

 increases the tension in the vessel on the cardiac side of the ob- 

 struction, and develops inflammation in the arterial coats, robbing 

 them of their cohesion and resisting power. 



Eppinger has shown the importance of infectious microbes in 

 weakening the arterial walls and predisposing to aneurism. 



The larval strongylus armatus already referred to is the most 

 potent factor in solipedes. They accumulate in the anterior 

 mesenteric artery, leading to clotting of the blood, inflammation of 

 the serous coat, and dilatation, so that in some verminous locali- 

 ties nearly every old horse shows a lesion of this vessel. 



All forms of arteritis, and disease of the vascular walls which 

 entail attenuation or weakening, predispose to aneurism. 



Of direct traumatism may be mentioned an aneurism of the 

 arch of the aorta in a horse struck b}^ a wagon pole, during a sharp 

 descent (Jacob), and two with aortic aneurism after violent blows 

 on the back with shafts of wagons. 



Symptoms. An aneurism within reach of the hand is to be 

 recognized primarily by the pulsation of tlie swelling synchron- 

 ously with the beats of the heart, and by a double rushing sound 

 with each beat of the heart, observed on auscultation. An ab- 

 scess over a large artery lying on a bone may puLsate but it is to 

 be distinguished b)^ the presence of a single in place of a double 

 rushing sound on au.scultation, by the possibility of causing more 

 or less complete collapse under pressure, and by the history of an 

 active phlegmonous inflammation followed by softening which 

 steadih^ extends from the centre of the previously dense mass. 



