312 DISEASES OF THE HEART. 



section of the work, hypertrophy of the unnaturally-tasked muscles of 

 respiration i^ one of the chief causes of the permanent expiratory con- 

 dition of the thorax of emphysema. We have not as yet any satisfac- 

 tory explanation of this fact ; but an interesting discovery has been 

 made, that a muscle, kept for some time in a state of tetanic contraction 

 by the application of electricity, maintains an increase of its volume 

 for several hours. 



Upon analysis of the conditions under which hypertrophy of the 

 heart arises, it will be found that most of them consist in disorders by 

 which both the vigor and the frequence of the cardiac contractions are 

 increased. (According to the measurements of J2izot y the thickness of 

 the ventricular wall continues to increase until late in life [unless gen- 

 eral marasmus arise], which is only ascribable to the" constant exercise 

 of the cardiac muscles.) 



It is admitted by the physiologists that there must be some regu- 

 lator of the action of the heart, which is still unknown to us, and by 

 which the energy of the organ is adjusted to meet the wants of the 

 system, and its activity increased as obstacles, which the heart has to 

 overcome, become augmented. It is a matter of importance, then, to 

 be able to demonstrate that hypertrophy of the heart occurs whenever 

 the function of the organ is permanently or repeatedly overtasked, 

 and when the resistance, which it should normally encounter, is in- 

 creased. To prove this is easy : 



1. Hypertrophy of the heart almost always accompanies abnormal 

 enlargement of its cavity (dilatation). When the heart is dilated, its 

 capacity is increased, and, as the organ cannot discharge its normal 

 load without expenditure of a certain degree of force, the effort re- 

 quisite for the expulsion of its abnormal increase of contents must be 

 proportionately greater, even though the resistance at the orifices and 

 in the arteries be normal. 



When we come to study pericarditis, we shall learn that there is a 

 form of hypertrophy of the heart which is purely the result of dilata- 

 tion. The immediate effect of the infiltration of the cardiac wall, which 

 takes place in this disease, is dilatation ; very soon, however, this is 

 followed by hypertrophy, although no fresh obstacle to the outflow 

 from the heart has arisen meantime. The first result of defective 

 closure of the valves of the heart is also dilatation ; and it is not until 

 afterward that hypertrophy develops in the portion of the heart imme- 

 diately before the diseased valve, and which is caused by the greater 

 effort now required to expel the increased amount of blood which the 

 heart contains. 



2. Hypertrophy of the heart accompanies stricture of its outlets, 

 and contraction of 1he great vascular trunks. No detailed explanation 



