148 CLINICAL VETERINARY MEDICINE AND SURGERY. 



acute form or of valvular lesions ; or, again, of some obstacle to circula- 

 tion reacting on the cardiac muscle, and forcing it to work more 

 actively- — a condition which first produces hypertroph}-, and sooner or 

 later changes of the nature of degenerative myocarditis. In these 

 cases functional hyperactivit}- is not acting alone, but is supplemented 

 by permanent passive congestion of the cardiac muscle, consequent on 

 the impediment to circulation. Under this double influence, changes 

 occur both in the fibres and the sarcolemma. The latter, being con- 

 tinuously irritated, responds by active proliferation (hyperplasia), while 

 the muscular fibres undergo granular degeneration, lose their striation 

 and contractility, and finally disappear under the pressure of the new 

 connective-tissue growth. Like the valvular lesions (of which it is a 

 consequence) this myocarditis is oftenest seen in the left heart. The 

 myocarditis following pulmonary emphysema, and specially marked in 

 the right heart, is due to a similar mechanism. In consequence of 

 stasis and increased pressure in the pulmonary artery, cardiac circulation 

 is impeded ; the blood escapes with difficulty from the coronary veins ; 

 the cardiac muscle therefore becomes congested, and the conditions 

 for degenerative and hyperplastic changes are at once realised. But, I 

 repeat, diffuse chronic myocarditis is almost always a deferred con- 

 sequence of infectious disease, during which the heart has been 

 affected ; it is the last stage of inflammation of the muscle of the heart. 



Partial or superficial myocarditis, developed by contiguity of tissue, 

 i. e. by extension to the cardiac muscle of inflammation at first localised 

 in the pericardium or endocardium, is sometimes seen. Though very 

 rare in the horse, it is common in the dog. 



In exceptional instances, myocarditis may result from the presence 

 of sclerostomata (parasitic worms) in one of the coronary arteries. I 

 reported a case in an ass suffering from chronic myocarditis, in which 

 the left coronary artery had become thrombosed, and close to its origin 

 showed an aneurism containing a dozen of these parasites. 



The anatomical changes shown by the cardiac muscle when the seat 

 of chronic inflammation result from two constantl}- associated factors, 

 the effects of which, however, may be developed to any degree, and 

 most frequently are unequally marked. In the majority of cases the 

 dominant lesion is the proliferation of interstitial connective tissue 

 leading to sclerosis of the myocardium ; in others it is the granulo-fatty 

 degeneration of the muscular fibres. 



When the contractile tissue is specially affected, the heart usually 

 remains of normal size, being simply softer and more relaxed, though 

 it may have undergone preliminar}- hypertrophy. After recent attacks, 



