HYPERTROPHY OF THE HEART. 313 



is needed to show that, under such circumstances, the resistance which 

 the organ has to surmount must be augmented. Obstruction of the 

 arterial outlets of the heart and insufficient calibre of the aorta, whether 

 congenital or acquired, is accompanied by hypertrophy of the ventri- 

 cles. Stenosis of the auricula-ventricular orifices is attended by hyper- 

 trophy of the auricles. 



3. Hypertrophy of the heart occurs in aneurism of the aorta and 

 of the pulmonary artery. It is a fact in physics that the resistance 

 encountered by a liquid, flowing through a tube, is increased if the 

 tube be contracted or expanded suddenly. Now, whenever the great 

 vessels which spring from the heart have undergone any considerable 

 aneurismal dilatation, particularly if the dilatation be of the circum- 

 scribed form, both of these conditions exist in the vessels, and thus an 

 additional tax is imposed upon the function of the heart. 



4. Hypertrophy of the heart is an accompaniment of obstruction 

 occurring in the range of the aortic current, or of the current of the 

 pulmonary artery. The greater and more extensive this obstruction, 

 so much the fuller must the aorta or pulmonary artery become, so 

 much the more intense the strain upon their coats, and so much the 

 greater the resistance which the heart has to surmount. 



The great majority of obstacles to the circulation, which give rise 

 to hypertrophy of the heart, are met with in the pulmonary circulation, 

 and hence cause hypertrophy of the right side of the organ. In a 

 previous section, a large number of diseases of the lungs and pleura 

 have come under our notice, in which hypertrophy of the right heart 

 has been shown to be not only a necessary consequence of anatomical 

 changes brought about by the affections of the lungs, but was even 

 counted as a symptom of these diseases themselves. In one case 

 evacuation of the pulmonary arteries is impeded, a part of its efferent 

 vessels, the capillaries of the air-vesicles, having perished through em- 

 physema. We have seen, too, how the capillaries are obliterated from 

 the shrunken tissues of a cirrhosed lung ; and that, in ple'irisy, some- 

 times one-half of the vessels into which the pulmonary artery should 

 discharge its contents are compressed and have become impervious. 

 But strain upon the pulmonary artery is not dependent upon diminu- 

 tion of the number of its efferent channels alone. A mere impediment 

 of the outflow of blood from the capillaries into the veins will produce 

 the same effect ; hence, to the catalogue of pulmonary diseases above 

 mentioned, we must add that of the affections of the left side of the 

 heart, which give rise to obstruction in the pulmonary vein. In treat- 

 ing of valvular deficiency of the left auriculo-ventricular passage, we 

 shall consider the subject of "compensating" hypertrophy of the right 

 ventricle more in detail. 



