660 ON THE ACTION OF STRYCHNIA ON THE HEART. 



are, nevertheless, most readily thrown into rhythmical action 

 "by the impressions conveyed to them from the other cavities. 

 In a normal condition of the heart, the venous sinus is the first 

 cavity to contract ; next comes the auricle, and next the ven- 

 tricle ; and a stimulus of contraction probahly proceeds from 

 one to the other along a channel furnished by the nervous 

 filaments which connect them. When the channel is suddenly 

 interrupted, as by ligature or division of the venous sinus, the 

 motor stimuli proceeding from the venous sinus to the auricle 

 and ventricle can no longer pass to them, and the reflex 

 impulses proceeding to their ganglia from tlie external and 

 internal surfaces of these cavities are insufficient to call them 

 into action. The auricles and ventricles, therefore, remain in a 

 state of quiescence for a longer or shorter period ; this qui- 

 escence, however, is not completely permanent. After a while 

 the ganglia seem to become adapted to the new, conditions. 

 Their sensibility, too, increases, and the stimuli proceeding to 

 them from the surface of the heart are sufficient to call them 

 into action. When the venous sinus is gradually removed 

 from the auricles and ventricle, instead of being suddenly 

 detached, time is afforded for this adaptation to take place 

 before the removal has been completely effected, and thus the 

 rhythm is not disturbed, as it is when the division is suddenly 

 made or the ligature suddenly applied. In these respects the 

 cardiac nervous system is analogous to the vaso-motor and 

 respiratory systems. The ordinary channels through which 

 the vaso-motor and respiratory centres and spinal cord are 

 called into action are the fibres which proceed to them from 

 the medulla oblongata. If these channels are suddenly inter- 

 rupted by section of the spinal cord at the occiput, those parts 

 of the vaso-motor and respiratory centres contained in the 

 spinal cord cease to act. The same is the case when a large 

 portion, but not the whole, of the respiratory centre in the 

 medulla is destroyed, as by division of one-half of the medulla. 

 When a large portion of this respiratory centre is thus 

 destroyed, the animal at once ceases to breathe, and remains in 

 this condition for many hours. If left to itself, death would of 

 course take place ; but if artificial respiration be maintained 



