THE CIRCULATION OF THE BLOOD. 297 



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. 



According to the first view the stimulus is neurogenic, according to the 

 second view myogenic. 



The presence of nerve-cells; their relation to the muscle-cells; the pro- 

 nounced rhythmic activity of the sinus and auricles in which the nerve-cells 

 are abundant; the feeble activity of the apex, in which they are wanting 

 these and other facts lend support to the view that the stimulus originates in 

 the nerve-cells. To them have been attributed the power of automatic 

 activity. 



The absence of nerve-cells in portions of the heart-muscle, which never- 

 theless exhibit rhythmic contractions for quite a long period of time; the 

 rhythmic beat of the embryonic heart before the migration of nerve-cells to its 

 walls shows that the stimulus does not necessarily originate in nerve-cells. 

 Moreover, Porter has conclusively shown that the apex of the dog's heart, 

 which is generally believed to be totally devoid of nerve-cells, can be made 

 to beat for hours by feeding it through its nutrient artery with warm defibrin- 

 ated blood. Unless it be assumed that the heart-muscle contracts auto- 

 matically, without a cause, it is a fair assumption that the exciting cause of 

 the contraction arises within the muscle-cells themselves, and that it is in 

 all probability the outcome of a reaction between the chemic constituents 

 of the blood or lymph on the one hand, and the chemic constituents of the 



