244 HANDBOOK OF PHYSIOLOGY. 



vagus and of the sympathetic are in opposite directions, and so if Gas- 

 kell's contention is correct that the negative variation of the muscle 

 current occurring on sympathetic stimulation is a sign of catabolism, the 

 result of vagus stimulation, viz., a positive variation of the muscle cur- 

 rent, may be supposed to indicate the complementary condition of ana- 

 bolism. 



3. The Amount of Blood Passing into the Heart's Cavities.~lt is found 

 that in the body at any rate the amount of blood which passes into the 

 cavities of the heart distinctly affects the strength of its beat. Thus if 

 from, any cause the blood is diminished the contractions become much 





Fig. 198. Plethysmograph. By means of this apparatus, the alteration in volume of the arm, 

 E, which is inclosed in a glass tube, A, filled with fluid, the opening through which it passes being 

 firmly closed by a thick gutta-percha band, p, is communicated to the lever, D, and registered by a 

 recording apparatus. The fluid in A communicates with that in B, the upper limit of which is 

 above that in A. The chief alterations in volume are due to alteration in the blood contained in the 

 arm. When the volume is increased, fluid passes out of the glass cylinder, and the lever, D. also is 

 raised, and when a decrease takes place the fluid returns again from B to A. It will therefore be 

 evident that the apparatus is capable of recording alterations of blood-pressure in the arm. Appa- 

 ratus founded upon the same principle have been used for recording alterations in the volume of 

 the spleen and kidney. 



more feeble, although they may possibly be increased in rapidity. Simi- 

 larly with 



4. Ihe Amount of Pressure to be Overcome. If the aortic pressure is 

 too low the muscle contractions of the heart is not so powerful or effec- 

 tive as if the pressure is normal, whereas too great arterial pressure may 

 be sufficient to delay if not to stop altogether the heart's beats, dilata- 

 tion of its cavities taking place and a condition of asystolism (Beau) 

 resulting. 



Another condition sometimes forgotten should be added as influenc- 

 ing the potency of the cardiac contraction, viz., the heart must have 

 sufficient room to contract, it must not be unduly pressed upon. 



5. The Coronary Circulation. The nutrition of the heart wall has 

 been fully investigated by Porter and others. The coronary arteries are 

 terminal arteries; that is, they do not permit the establishment of a 

 collateral circulation when one of their branches is blocked. If the 



