430 THE CIRCULATORY SYSTEM. 



5. Dilatation of the ventricles, from any cause, is frequently followed 

 by hypertrophy. 



6. Pericarditis may lead to hypertrophy by inducing softening and 

 dilatation of the ventricles, or by leaving adhesions which obstruct the 

 heart's action. Chronic myocarditis also may lead to hypertrophy. 



Finally, for some cases of hypertrophy no satisfactory cause can be 

 found. ' 



It should be borne in mind that an increase in the amount of fat in 

 and about the heart may make the organ appear larger, when there may 

 be actually a considerable decrease in the amount of muscle tissue. 



DILATATION. 



Dilatation may be combined with hypertrophy active dilatation; or 

 there may be no increase of muscle tissue, but a thinning of the walls 

 proportionate to the dilatation of the cavity passive dilatation. 



Either one or all of the heart cavities may be dilated, the auricles 

 most frequently ; next the* right ventricle ; least often the left ventricle. 



Active dilatation has been considered under hypertrophy. 



Passive dilatation may be associated with : 



1. Changes in the valves. Mitral or aortic stenosis or insufficiency 

 may lead to dilatation of the auricles and right ventricle. Pulmonary 

 stenosis or insufficiency may lead to dilatation of the right auricle and 

 right ventricle. Aortic insufficiency, with or without stenosis or mitral 

 insufficiency, may lead to dilatation of the left ventricle. Dilatations 

 under these conditions are often succeeded and compensated for by 

 hypertrophy of the heart walls. 



2. Changes in the muscle tissue of the heart walls. Serous infiltra- 

 tion from pericarditis, myocarditis, fatty degeneration and infiltration, 

 atrophy of the muscle fibres, may all lead to dilatation. 



3. A heart which is already hypertrophied may, from degeneration 

 of the muscle, become dilated. 



4. Acute exudative inflammations of the lungs and acute pleuritic 

 exudations, by rendering a large number of vessels suddenly imperme- 

 able to the blood current, may produce sudden stasis in the pulmonary 

 artery and dilatation of the right heart. 



5. There are curious and often serious cases of acute and chronic 

 dilatation of the ventricles for which no mechanical explanation is found. 



CHANGES IN THE HEART VALVES. 



Fenestration of the valves is usually a change of no practical impor- 

 tance. It occurs very frequently in the aortic and pulmonary valves. The 

 valves may be thinner than usual, and close to their free edges are small 

 slits extending from the centre to the attached edges of a leaf (Fig. 208). 



1 Howard's table of 105 cases of cardiac hypertrophy shows its association with 

 arterio- sclerosis in 59 per cent; with nephritis in 18.4 per cent; with valvular lesion in 

 12.4 per cent. Johns Hopkins Hospital Reports, vol. iii., p. 265. 



