26 THEORY AND PRACTICE 



most common in the human. ' In this 

 condition the chalky portion crumbles 

 off as in leprosy. It never occurs in 

 horses. 

 A common example of dry gangrene may be seen in a sitfast 

 on the horse. It is caused by the screws of the saddle extending 

 too far down and producing pressure upon the skin; the part 

 loses blood and dries, becoming gangrenous and insensible to 

 pain. It does not slough, but finally produces a zone of inflam- 

 mation around it. The only way to cure it is to remove the dead 

 part and to remove the cause. 



THROMBOSIS AND EMBOLISM. 



A thrombus is a plug or clot in a vessel remaining at the 

 point of formiation. An embolus is a clot or other plug brought 

 bv the blood current from a distant vessel and forced into a 

 smaller one, thereby obstructing the circulation. 



Thrombosis is the coagulation of the blood within the heart 

 or blood vessels during life — it is the plugging of a vessel by 

 a thrombus. Embolism is produced by an embolus, which is a 

 body floating in the blood until it lodges in a vessel too small for 

 it to pass through. In either case active congestion would be 

 produced by increased blood pressure. When a thrombus forms 

 in a part wdiich has no collateral circulation, the part dies from 

 inanition. In the brain there is no free anastomosis between the 

 vessels and if emboli lodge in them the tissues soon degenerate 

 and die. 



In embolism a clot has formed perhaps on the valve of the 

 heart, or a little fibrin lodged among the chordae tendine?e : a 

 little piece floats off to some distant vessel and lodges, forming 

 an embolism. An illustration of this may be seen in a horse 

 which suddenly becomes lame ; he drives all right for two or three 

 miles and then suddenly goes lame on one leg, in a few minutes 

 is practically on three legs. He sweats profusely and the af- 

 fected leg is cold. After rest he is all right, but this experience 

 occurs again and again until he is practically useless. The cause 

 is an arterial embolus. During exercise the collateral circulation 



