1892.] Physiology and Pathology of the Mammalian Heart. 441 



tractions, so that the force of the heart's contractions is to a certain 

 extent regulated automatically by changes in the blood pressure in 

 the aorta, which is one of the variable quantities affecting the work 

 of the left ventricle. 



Change of the volume of blood in the body affects greatly the con- 

 traction volume and output of the heart. Injections into the veins of 

 a volume of defibrinated blood equal to one-tenth of the total blood 

 in the body may double the output. It is important to note here 

 that there is no increase in the strength of the ventricular contrac- 

 tions; increase in the work, therefore, of the ventricles due to increase 

 in the output has no tendency to automatically increase the force of 

 the ventricular contractions, as is the case with rise of pressure in the 

 systemic arteries. We refer to the bearing of this in cases of 

 plethora. 



Increase of the watery constituents of the blood increases the con- 

 traction volume and output to the same extent (though only tempor- 

 arily) as does transfusion of blood, but acts more unfavourably on 

 the heart, seeing that the work done by the ventricles is increased, 

 while the nutritive value of the blood supplied to the coronaries is 

 diminished. 



The increased output of the heart both in plethora and in hydraemia 

 is due to rise of pressure in the systemic veins increasing the volume 

 of blood which enters the right ventricle during diastole. We refer 

 to the bearing of these facts upon the treatment of chlorosis and 

 heart disease. 



In Section XIV we consider the limits of the power of the heart to 

 perform the work thrown upon it, and show that in strictly physio- 

 logical conditions, and in spite of the beautiful mechanism by which 

 the force of the ventricular contraction is regulated, the heart, like the 

 voluntary muscles of the body,is liable to fatigue when the work thrown 

 upon it greatly exceeds that required to maintain the circulation under 

 ordinary circumstances. We take as example the increased work 

 thrown upon the organ during active muscular exertion, and show 

 that exertion and endurance of fatigue are limited mainly by the 

 limited power of the heart to continue supplying the increased 

 amount of blood which is required by the acting voluntary muscles. 

 We show that those luxuries which are forbidden or limited in 

 "training," and which are known to hinder prolonged exertion, such 

 as water, alcohol, tobacco, caffein, all directly weaken the force of the 

 heart's contractions, and, in the case of water, place the organ under 

 a disadvantage ; also that fatigue of the heart leads to dilatation of 

 the organ. 



On comparing the power of fatigued ventricles to carry on increased 

 work, as compared with well-nourished unfatigued ventricles, it is 

 found that not only is the strengthening effect of the augmeutor 



2 H 2 



