ANEUEISM. 419 



ANEUKISM. 



We niean by this a tumour formed by coagulated arterial 

 blood within a dilated artery, or between the several coats 

 of such a vessel, or externally to it. There are false and 

 true aneurisms. In the latter the arterial coats are wholly 

 or partially entire, and the former consist in accumulation of 

 blood around the vessel. Aneurisms are also termed spon- 

 taneous or traumatic, according to their mode of origin; 

 they may be external or internal. 



Spontaneous aneurisms may consist in dilatations of a con- 

 siderable extent, or they may be circumscribed; the coats 

 may be intact and even thickened. In other instances the 

 inner coats have become attenuated, the external coat is forced 

 outwards. When the aneurism has commenced to form, it 

 enlarges rapidly and becomes circumscribed. Layers of 

 blood clot, undergoing progressive discolouration, accumulate, 

 and a narrow aperture alone communicates in many cases 

 with the current of blood. As the aneurism grows old, the 

 coats of the artery become indurated, calcified, and liable to 

 burst. 



On the production of aneurism, Virchow says that " if the 

 elasticity of the vessel be considerably diminished, without 

 its becoming stiff and iminoveable (from calcareous incrusta- 

 tions) in the same degree, the dilatation which it undergoes 

 from the pressure of the blood, is not again compensated; the 

 vessel remains in a dilated condition, and thus are gradually 

 produced the well-known forms of ectasis, such as we are 

 familiar with in the arteries under the name of aneurisms, 

 and in the veins under that of varices. In these processes 

 we have not so much, as has been represented of late, to deal 

 with primary disease of the inner coat, as with changes which 

 are situated in the elastic and muscular middle coat." 



Spontaneous aneurism is chiefly seen in the horse and 



