3o6 TEXT-BOOK OF PHYSIOLOGY 



develops a rhythm of its own, though it never attains that of the auricle. 

 From these facts it is probable that in each division of the heart a stimu- 

 lus similar to that acting in the sinus is developed when the heart 

 chambers are separated one from the other. 



4. Tonicity. Tonicity may be denned as a condition of muscle material 



characterized by a slight degree of contraction which varies in extent, 

 however, from time to time under physiologic conditions. Whatever 

 the cause of the tonicity may be in any given form of muscle, the 

 slight degree of contraction which characterizes it not only resists undue 

 extension but permits of a quicker response to the action of a stimulus 

 and a more effective performance of work. The heart-muscle, like 

 the skeletal muscle, maintains continuously a certain degree of contrac- 

 tion, which not only prevents undue expansion of the heart during the 

 period of diastole, but increases its efficiency as a pumping organ at the 

 beginning and during the systole. The results obtained from subjecting 

 the heart muscle to the action of a calcium salt renders it probable that the 

 tonicity is in large measure the result of or is associated with the action of 

 such a salt (see page 307). The tonicity may, however, be increased 

 or decreased by the action of various external agents. Thus the passage 

 of dilute solutions of various drugs e.g., alkalies, digitalis through 

 the cavities of the excised frog heart will so increase the tonicity, or the 

 contractile power, that complete relaxation is prevented, until finally 

 the heart comes to a standstill in the condition of systole. The passage 

 of dilute solutions of lactic acid, muscarine, etc., through the heart will, 

 on the contrary, so decrease the tonicity or the contractile power that 

 the normal contraction is not attained. The relaxation therefore 

 gradually increases until the heart finally comes to a standstill in the 

 condition of extreme diastole. In the first instance the tonicity is said 

 to be increased; in the second instance, decreased. 



5. Automaticity. Automaticity may be defined as the power of maintain- 



ing activity by a self-acting cause or the power of acting independent 



of external causes. Inasmuch as the heart continues to contract in 



a perfectly rhythmic manner after removal from the body and apparently 



- without the aid of an external stimulus, it is said that the heart-muscle 



is automatic or spontaneous in action. Strictly speaking, however, this 



is not the case, for the reason that all movement, that of the heart 



included, is the resultant of the action of natural causes though their 



true nature may be beyond the reach of present methods of investigation. 



The Nature of the Stimulus. As the heart continues to beat after 



removal from the body, it is evident that the stimulus does not originate in 



the central nerve system but in the heart itself. Two views have been held 



as to its origin and nature: 



1. That it originates in the nerve-cells found in various parts of the heart- 



muscle; that it is a nerve impulse rhythmically and automatically 

 discharged by these cells and transmitted by their axons to the heart- 

 muscle cells. 



2. That it originates in the muscle-cells themselves; that it is chemic in 



character and due to a reaction between the chemic constituents, 

 organic and inorganic, of the muscle-cells and those of the lymph by 

 which they are surrounded. 



