578 CIRCULATION OF BLOOD AND LYMPH, 



dominal viscera, by a blow upon the viscera, for example, or 

 by sudden distention of the stomach. In man similar results 

 are noticed very frequently. Acute dyspepsia, inflammation of 

 the peritoneum, painful stimulation of sensory surfaces, — the 

 testes, for instance, or the middle ear, — may cause a marked slowing 

 of the heart, — a condition designated as bradycardia. What 

 takes place in all such cases is that the afferent impulses carried 

 into the central nervous system reflexly stimulate the nerve cells 

 in the medulla which give origin to the inhibitory fibers. These 

 cells form a part of the great motor nucleus (N. ambiguus) from 

 which arise the motor fibers of the vagus and the glossopharyngeus. 

 The particular group of cells from which the inhibitory fibers to the 

 heart originate has not been delimited anatomically. Efforts have 

 been made to locate them by vivisection experiments, but this 

 method has shown no more perhaps than that they are found in the 

 region of origin of the vagus nerve. Physiologically, however, this 

 group of cells forms a center which is of the greatest importance in 

 controlling the activity of the heart. It is designated, therefore, as 

 the cardio-inhibitorji center. We may define the cardio-inhibitory 

 center as a bilateral group of cells lying in the medulla at the level of 

 the nucleus of the vagus and giving rise to the inhibitory fibers 

 of the heart. The two sides are probably connected by commis- 

 sural cells or else each nucleus sends fibers to the vagus of each 

 side. Through this center all reflexes that affect the heart by way of 

 the inhibitory fibers must take place. These reflexes may be occa- 

 sioned by incoming sensory impulses through the spinal or cranial 

 nerves, or by impulses coming down from the higher portions of 

 the brain. The center may also be stimulated directly, either by 

 pressure upon the medulla, which may give rise to slow heart beats 

 or, as they are sometimes called, vagal beats, or by changes in the 

 composition of the blood. With regard to the reflex stimulation of 

 this center it is important to bear in mind the general physiological 

 rule that afferent impulses may either excite or inhibit the activity 

 of nerve centers. In the former case the heart rate would be 

 slowed, in the latter case it would be quickened if the center were 

 previously in a state of activity. 



The Tonic Activity of the Cardio-inhibitory Center. — ^The 

 cells of the cardio-inhibitory center are in constant activity to a 

 greater or less extent. As a consequence, the heart beat is kept con- 

 tinually at a slower rate than it would normally assume if the 

 inhibitory apparatus did not exist. This tonic activity of the vagus 

 is beautifully exhibited by simple section of the two vagi, or by inter- 

 rupting, in some other way — cooling, for example — the connection 

 between the center and the heart. When the two vagi are cut the 

 heart rate increases greatly and the blood-pressure rises on account 

 of the greater output of blood in a unit of time (Fig. 242). Section 



