THE CIRCULATION OF THE BLOOD 323 



discharge of nerve impulses which, conducted to the heart by the pre- 

 ganglionic and post-ganglionic sympathetic fibers, cause an acceleration in 

 the rate or an augmentation in the force, or both, of the heart-beat. It is 

 also generally believed since the publication of Hunt's investigations that 

 this center is in a state of tonic activity. This is shown by the fact that after 

 the division of the vagus nerves and the removal of all possible inhibitor 

 influences, division of the sympathetic nerves or extirpation of the stellate or 

 inferior cervical ganglion, is yet followed by a decrease in the rate of the heart- 

 beat. After division of the sympathetic nerves and the removal of accelerator 

 influences it is also easier to bring about inhibition through vagus stimulation. 



The Factors which Determine the Activity of the Cardio-accelera- 

 tor Center. The question has been raised as to whether the tonic activity 

 of this center is maintained by central or peripheral stimuli, i.e., whether it 

 is maintained by causes within itself, the result of an interaction between the 

 constituents of the cell substance and those of the surrounding lymph, or 

 whether it is maintained by nerve impulses reflected to it through various 

 afferent or sensor nerves. Inasmuch as there is no way of determining 

 whether the causes are central, except by dividing all afferent nerves, it is 

 impossible to state how much influence is to be attributed to this factor. 

 On the contrary, though it is readily demonstrable that stimulation of many 

 afferent nerves will cause an acceleration of the heart it cannot be stated 

 positively that this is the result of a reflex stimulation of the accelerator center. 

 Though earlier investigators believed this to be the correct interpretation, 

 the more recent experiments of Hunt apparently disprove it; for this investi- 

 gator has shown that if the vagus nerves are divided it is impossible to pro- 

 duce reflex acceleration of the heart. His conclusion, confirming that of 

 others, is that cardiac acceleration is the result of an inhibition of the cardio- 

 inhibitor center. A freer play to the tonic activity of the accelerator center 

 would thus be made possible. 



The Cardio-inhibitor Center. The collection of nerve-cells from which 

 the small efferent fibers of the vagus nerve arise is known as the cardio-in- 

 hibitor center. 1 It is situated in the medulla oblongata or more exactly in 

 the gray matter beneath the floor of the fourth ventricle near the tip of the 

 calamus scriptorius. It is in all probability a part of the nucleus ambiguus. 



From the experiments which have been made on the vagus inhibitor 

 apparatus in its entirety it is believed that the function of this center is the 

 discharge of nerve impulses which conducted to the heart by the vagus 

 fibers cause an inhibition of its beat of greater or less extent. In the dog, 

 and probably in many other mammals, this center exerts a more or ^less 

 constant inhibitor or restraining influence on the heart's activity. This is 

 indicated by the fact that the rate of the beat is very much increased by 

 simultaneous division of both vagi. The degree of the inhibition which this 

 center exerts varies greatly, however, in different animals. In the cat and 

 in the rabbit the inhibitor control is normally so slight that there is but a 

 relatively slight increase in the rate of the beat after division of the vagi. 

 The tone of the vagus in these animals is, therefore, said to be j 

 feeble. In human beings the tone of the inhibitor apparatus is po 



i Inasmuch as the nerve system is bilaterally arranged there are of course two nerve-centers 

 here as elsewhere, one on each side of the median line Aough they are associated in action b, 

 either commissural fibers or by a decussation of some of their efferen 



