3i6 Veterinary Medicine. 



veloped legs of the ballet dancer or the brawny arm of the 

 blacksmith. All alike occur in accordance with a general law 

 that whenever there is habitually demanded of any organ an un- 

 usual activity of function, which stimulates without exhausting 

 its power, nature adds to the active element of such organ till the 

 required labor can be accomplished without the overwork of any 

 particular part. 



Keeping this in view we can easily explain the increase of one 

 part of the heart without immediate implication of another. The 

 ventricles are more commonly enlarged than the auricles because 

 upon them devolves the work of overcoming the obstruction, 

 whether this exists in the lungs or the system at large. The 

 auricles fulfill little more than a passive function in receiving the 

 blood from the veins during the contraction of the ventricles and 

 allowing it to pass down into these when their relaxation takes 

 place. The closure of the auriculo-ventricular valves during the 

 ventricular contraction protects the auricles from the internal ten- 

 sion to which the lower part of the heart is subjected and thus 

 all tendency to increase is obviated. 



The hypertrophied part corresponds to the locality of the ob- 

 struction. If it exists in the lungs (heaves, consumption, hepa- 

 tisation, chronic bronchitis), pulmonary artery, its valves at its 

 origin from the heart, or if it consists in contraction of that ori- 

 fice, the enlargement takes place primarily in the right ventricle, 

 the right auricle remaining unchanged so long as the auriculo- 

 ventricular valves act perfectly. The ventricle, however, tends 

 to dilate as well as enlarge in thickness of walls, and as soon as 

 this dilatation has proceeded so far as to widen the orifice between 

 the auricle and ventricle and render its valves insufficient, the 

 auricle also begins to dilate and its walls often increase in thick- 

 ness. But the vicious chain does not end here. Should the 

 animal survive and the original obstruction persist, the veins 

 throughout the system become habitually congested because of 

 the reflux of blood from the right auricle and ventricle, dropsies 

 appear in different parts, the congestion of the veins is continued 

 through the capillary blood-vessels to the arteries, the difficulty 

 of propelling the blood comes to be experienced by the left ven- 

 tricle and a corresponding series of morbid changes taking place 

 on that side, as have already ensued on the right, the vicious 



