330 DISEASES OF THE HEART. 



CHAPTER II. 



DILATATION OF THE HEART. 



IN excentric hypertrophy of the heart, the cavities of the organ 

 become enlarged, but, as its walls at the same time are thickened by 

 development of their muscular substance, the heart does not suffer any 

 impairment of its functional power, which, indeed, is augmented. Al- 

 though excentric hypertrophy has been called " active dilatation " by 

 the pathological anatomists, yet, from a clinical point of view, it can- 

 not be regarded as a dilatation of the heart. According to the gen- 

 eral usage both of physicians and laity, the term dilatation is exclu- 

 sively applied to a morbid condition of the heart, in which its cavities 

 are enlarged, but in which there is no corresponding growth of the 

 muscular substance of its walls, so that the contractile power of the 

 organ is diminished. This condition corresponds to the " passive dilata- 

 tion " of the pathological anatomists. 



There are three recognizable forms of cardiac dilatation, although 

 the transition of the first variety into the second is not abruptly defined : 



1. The cavity is dilated, the wall of the heart retaining its normal 

 tliickness, and being merely comparatively too thin. 



2. The cavity is dilated and the cardiac wall is positively thinner 

 than is normal. 



3. The cavity is dilated, while the heart- wall is thickened ; not, 

 however, by augmentation of its muscular substance, but through a 

 spurious hypertrophy. 



ETIOLOGY. The causes of dilatation of the heart are 

 1. When the organ is subjected to an unnaturally severe internal 

 pressure during its diastole, causing its wall to yield to a certain de- 

 gree, the fact, that contractions of the orifices of the heart, and other ob- 

 stacles to the circulation, cause dilatation of the portion of the organ from 

 which the efflux of blood is impeded, might give rise to the erroneous 

 supposition that an abnormal resistance, encountered by the heart 

 during its systolic movement, may result in dilatation. It is manifest, 

 however, that, the moment that the contractile force of the heart be- 

 comes incapable of overcoming the resistance opposed by its contents, 

 and it yields to internal pressure, the circulation of the blood will 

 stop. The explanation of the mode in which dilatation is produced, 

 when one of the cardiac orifices has become contracted, is as follows : 

 The first consequence of an obstacle to the circulation is, that the 

 affected cavity is incompletely emptied of its blood. The gush of 

 blood which enters the heart upon diastole, instead of finding the cav- 

 ity empty, finds it almost as full at the beginning of the diastolic move- 



