



THE AUTOMATICITY OF THE HEART ' 207 



the bundle system. It is approximately 0.16 seconds. The time at 

 which the contraction stimulus arrives at different points on the ventri- 

 cles, measured from the moment of the beginning; of the R wave is illus- 

 trated in Figure 176^. 



This newer conception of the heart's differ- 



entiations gives to the true cardiac muscle a HHE! *i ~' 



position in energy production under the direct | : |* 



control of the conducting system for its coor- ^M 1 



dination. To the conducting system is rele- 

 gated the function of stimulus production and 



a rapid distribution of the stimulus that still 1?: 



~ ~ 

 preserves a mechanically efficient sequence. 



The function of the nerve elements by 



this view is neither to initiate rhythm pro- ; .- ; , -^ 



duction nor control the orderly sequence of ? 



the beats but to regulate the whole organ as 

 regards its four main functions, namely, rhythm, 

 conduction, energy production, and irritability. 

 The details of this regulation are discussed 

 later. ^llW^^^^^f^ Hv 



Relation of Rhythm to Nutrition. The 

 whole heart, like the muscular parts of which 

 it is composed, responds delicately to its con- 

 dition of nutrition. In the frog and the turtle 



hearts the muscular fibers are brought in inti- ^ 



mate contact with the blood contained within r ? 



the cavities. In the mammalian heart, on the 



- 



other hand, a distinct system of vessels, the r ^OT$^i? 



coronary vessels and the vessels of Thebesius, ;3S^^ ^P^ : ^ 



supply blood to the walls. If the heart is sup- ^Jjjt'V ^ 



plied with nutrient fluid similar to its normal >igjf ;. 



blood, and with proper aeration to insure plenty 



of oxygen, it contracts with a strong rhythm for 



many hours. This rhythm, however, responds 



quickly to changes in the composition of the '" ;; "c^ : fg "..- 



nutrient fluid. An abundant supply of oxygen :L " 



is absolutely necessary to the maintenance of 



rhythm in the mammalian heart, though the !p F : ?"-- .^B "c 



heart, especially a cold-blooded heart, will con- v j. ^x-. - : '.?,.* <i- 



tract for a time in an atmosphere of hydrogen. ^^^.^ ; ^ ^ K"" 1 



No doubt the organic constituents of blood are '^^fe^' %> 



very essential to the prolonged maintenance 



of rhythm in the heart, but the heart is not 



dependent on these ingredients for its stimulus 



