518 CIRCULATION OF BLOOD AND LYMPH. 



viscera, and most observers state that the heart may be reflexly 

 inhibited most readily by stimulation of the sensory surfaces of 

 the abdominal viscera, by a blow upon the viscera, for example, 

 or by sudden distension of the stomach or sudden emptying of 

 the bladder. In man similar results are noticed very frequently. 

 Acute dyspepsia, inflammation of the peritoneum, painful stimu- 

 lation of sensory surfaces, the testes, for instance, or the middle 

 ear, may cause a marked slowing of the heart, a condition des- 

 ignated as bradycardia. What takes place in all such cases is that 

 the sensory 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-inhibitory 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- 



