THE CIECULATION OP THE BLOOD. 



253 



contracted, the others must necessarily follow ; hence the rapid 

 onset of the ventricular after the auricular contraction in the 

 mammal, and the long wave of contraction that seems to pass 

 evenly over the whole organ in cold-hlooded animals. 



The basis of all these factors is to be sought finally in the 

 natural contractility of protoplasm. A heart in its most de- 

 veloped form still retains, so to speak, the inherited but modified 

 Amceba in its every cell. 



Whether the intrinsic nerve-cells of the heark take any share 

 directly in the cardiac heat must be considered as yet undeter- 

 mined. Possibly they do modify motor impulses from nerves, 

 while again it may be that they have an influence over nutri- 

 tive processes only. The subject requires further study, both 

 anatomical and physiological. 



INFLUENCE OP THE VAGUS NERVE UPON THE HEART. 



The principal facts in this connection may be stated as fol- 

 lows, and apply to all the animals thus far examined : 



1. In all cases the action of the heart is modified by stimu- 

 lation of the medulla oblongata or the vagus nerve. 



2. The modification may consist in prompt arrest of the 

 heart, in slowing, in enfeeblement of the beat, or a combination 

 of the two latter effects. 



3. After the application of the stimulation there is a latent 



JljlJlilMlili^^ 



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Pig. 214.— Inhibition of frog's heart by stimulation of the vagus nerve. To be rend 

 from right to left. The contractions of the ventricle are registered by a simple 

 lever resting on it. The interrupted current was thrown in at a. Note that one 

 beat occurred before arrest (latent period), and that «'hen standstill of the heart 

 did take place it lasted for a considerable period (Foster). 



period before the effect is manifest, and tlie latter may outlast 

 the stimulation by a considerable period. 



