ENLARGEMENT OF THE HEART. 569 



dilatation, at others the dilatation is greater than the hyper- 

 trophy. 4. Another form is mentioned by some, but its 

 existence is doubtful, apart from congenital malformation; 

 i.e., hypertrophy with diminished cardiac capacity — concentric 

 hypertrophy. 



Causation. — Hypertrophy, with or without dilatation, may in 

 a large view of the causation be regarded as a comjiensatory 

 condition, and as the effort of the main organ of the circula- 

 tion to overcome some existing obstacle. 



1. It may ajipear in what may be regarded as an idiopathic 

 form in animals which have for a lengthened period been 

 subject to continuous and severe exertion. In horses which 

 have been hunted for some years simple hypertrophy, or 

 hypertrophy with dilatation, is of frequent occurrence, not 

 appearing suddenly, but gradual in development. 



2. It is attendant on direct obstruction to blood-flow (a) oc- 

 curring at the cardiac orifices and large vessels, chiefly at 

 the mitral and aortic openings ; (6) in the pulmonary circu- 

 lation from structural changes in lung-tissue, as in bronchitis 

 and emphysema ; (c) occasionally m the systemic circulation, 

 from impediment in different organs, particularly when suf- 

 fering from fibrosis and other changes in peripheral vessels. 



3. Dilatation with hypertrophy is very Hable to occur from 

 internal pressure during systolic relaxation, when, from aortic 

 or mitral insufficiency, regurgitation takes place. 



4. Enlargement of the same character may arise from weak- 

 ening of the muscular structure following inflammatory 

 action. 



Anatomical Characters. — Of these, the chief are increase in 

 weight and increase in bulk, the relative extent of these being 

 in proportion to the ascendency of hypertrophy or of dilata- 

 tion. 



With many, change of form or shape of the organ is a con- 

 spicuous feature, the character and extent depending upon the 

 parts actually affected; ordinarily it is more globose than 

 natural. In the intimate structure, unless degenerative changes 

 have occurred, the appearance of the muscular substance is a 

 darkening or heightening in colour, and either an increased 

 development of the minute fibres, or the production of new 



