242 DISEASES OF THE HORSE. 



ATHEROMA. 



Atheroma is a direct result of an existing chronic endarteritis, the 

 lining membrane of the vessels being invariably involved to a greater 

 or less degree. It is most frequently found in the arteries, although 

 the veins may develop an atheromatous condition when exposed to any 

 source of j^rolonged irritation. Atheroma may affect arteries in any 

 part of the body; in some instances almost every vessel is diseased, 

 in others only a few, or even parts of one vessel. It is a very common 

 result of endocarditis extending into the aorta, which we find perhaps 

 the most frequent seat of atheroma. As a result of this condition the 

 affected vessel becomes impaired in its contractile power, loses its 

 natural strength, and, in consequence of its inability to sustain its 

 accustomed internal pressure, undergoes in many cases dilatation at 

 the seat of disease, constituting aneurism. In an atheromatous vessel, 

 calcareous deposits soon occur, which render it rigid, brittle, and sub- 

 ject to ulceration or rupture. In such vessels the contractility is de- 

 stroyed, the middle coat atrophied and beyond repair. Atheroma in 

 the vessels of the brain is a frequent cause of cerebral apoplexy. No 

 symptoms are manifested by which we can recognize this condition 

 during life. 



CONSTRICTION OF AN ARTERY. 



This is usually the result of arteritis, and may partly or wholly be 

 impervious to the flow of blood. When this occurs in a large vessel 

 it may be followed by gangrene of the parts; usually, however, col- 

 lateral circulation will be established to nourish the parts previously 

 supplied by the obliterated vessel. In a few instances constriction of 

 the aorta has produced death. 



ANEURISM. 



Aneurism is usually described as true or false. True aneurism is a 

 dilatation of the coats of an artery over a larger or smaller part of its 

 course. Such dilatations are usually due to chronic endarteritis and 

 atheroma. False aneurism is formed after a puncture of an artery by 

 a dilatation of the adhesive lymph by which the puncture was united. 



Symptoms. — If the aneurism is seated along the neck or a limb it 

 appears as a tumor in the course of an artery and pulsating with it. 

 The tumor is round, soft, and compressible, and yields a peculiar 

 fluctuation upon pressure. By applying the ear over it a peculiar 

 purring or hissing sound may sometimes be heard. Pulsation, syn- 

 chronous with the action of the heart, is the diagnostic symptom. It 

 is of a slow, expansive, and hea^^ character, as if the whole tumor 

 were enlarging under the hand. Aneurisms seated internally may 

 occupy the cavity of the cranium, chest, or abdomen. As regards the 



