60 A MANUAL OF VETERINARY PHYSIOLOGY 



during life. As evidence of the gross strain to which horses are 

 exposed, ruptures of the heart are by no means uncommon. It is 

 strange they are not more frequent. They probably would be but 

 for the saving cause that degenerations of the heart substance are 

 rare. When the heart ruptures, it gives way in the auricle, where 

 the wall is thinnest — so thin, indeed, that in certain parts of the 

 auricle daylight may easily be seen through the tissue. It is gener- 

 ally the right and not the left auricle which suffers, showing how 

 great is the resistance offered by the pulmonary vessels as the result 

 of engorgement due to severe work. 



Valvular disease is not unknown, but so rare that probably there is 

 no practitioner with a large experience in the examination of horses 

 for soundness who ever thinks of examining the heart ! On the 

 other hand, irregularities in the heart's action are very common, 

 frequently purely functional in character, unassociated with organic 

 change, and do not interfere with the usefulness of the animal. A 

 horse condemned for heart disease on the strength of an intermittent 

 pulse may remain a living reproach to the practitioner. 



The views regarding heart disease in man have within the last 

 few years been undergoing profound modification at the hands ot 

 Mackenzie, Lewis, and other workers. It is too much to expect 

 that the complete revolution in doctrine which these necessitate will 

 be at once brought about ; the process must be gradual, but when 

 accomplished it will be found that very little modification from 

 the views now put forward will be found necessary. Briefly, the 

 modern view of the causes of heart failure looks to the heart muscle 

 itself as the prime seat of disorder, while the coarser and more 

 obvious conditions, such as murmurs and irregularities, may not 

 even be pathological^ or, if pathological, are of secondary interest 

 to the all-important inefficiency existing in the heart muscle. Mac- 

 kenzie has recently* presented a statement of the modern views of 

 heart trouble in man, of which the following is a brief epitome : 



The popular conception of heart failure is associated with valvular 

 trouble, the thickened and shrunken valve producing incompetence, 

 with leakage from the ventricle, distension of the auricle, stasis in 

 the lungs, back pressure into the veins, with consequent dropsy. 

 Functional murmurs may exist in a perfectly healthy heart under 

 physiological dilatation ; a tricuspid regurgitation may be regarded 

 as a safety-valve function, as it is in certain diving animals ; and a 

 mitral regurgitation may exist for fifty years without crippling the 

 heart in its work. Irregularities of the heart may not only be 

 normal, but even, within certain limits, considered as evidence of 

 the integrity of the heart muscle. 



These unorthodox views from the mouth of one who has confined 

 his attention to the study of heart trouble for many years, must 

 cause even the most sceptical to pause. According to Mackenzie, 

 the essential causes of heart trouble in man are due to auricular 

 fibrillation, a condition which alters the whole aspect of the 

 mechanism of heart failure. To the phenomenon of fibrillation 

 attention has been drawn at p. 47. The muscle fibres, instead of 

 contracting in a normal and orderly manner, contract irregularly, 

 rapidly, and independently, so that the chambers not only cease to 

 contract as a whole, but actually stand still, while all their fibres are 

 in incessant movement. When this condition occurs in the ven- 



* ' Heart Failure,' James Mackenzie, M.D., LL.D., British Medical 

 Journal, April 8, 191 1, p. 793. 



