188 CIRCULATION OF THE BLOOD 



porarily abolished exactly at the injured place (F. B. Hofmann), which however 

 could only be explained by an action of inhibitory fibers on motor fibers. Such 

 effects are entirely unknown elsewhere and are therefore extremely improbable 

 here. The phenomenon presents no special difficulty, on the other hand, if we 

 suppose that the transmission is purely muscular, and has been rendered im- 

 possible by the action of the inhibitory nerves on the injured muscle. 



The synchronism of the two ventricles is not effected by simultaneous im- 

 pulses from the auricles, but by their muscular or nervous connection with each 

 other. For if the two be separated from one another, but be left in connection 

 with their respective auricles, the synchronism is broken and each beats in its 

 own rhythm (Porter). 



/ 11. THE EFFERENT CARDIAC NERVES 



The activity of the heart is controlled by impulses from the central nervous 

 system brought to it over the vagus and sympathetic fibers. Afferent nerves 

 also pass to the brain from the heart, and these influence both the heart itself 

 and the blood vessels of the general system reflexly. The rich supply of nerve 

 fibers to the ultimate muscle fibers of the heart are the terminal branches of 

 these same nerves. 



The importance of these regulatory influences can scarcely be overestimated. 

 This is well shown by the following observations of Friedenthal on a dog, whose 

 extracardiac nerves were all cut, the afferent fibers from the lungs, and the fibers 

 to the stomach and the oesophagus being preserved on one side. The animal, 

 which had survived the last operation for more than eight months and had then 

 succumbed to acute strophanthus poisoning, showed in the meantime on super- 

 ficial examination scarcely any abnormality. The number of heart beats, for 

 example, was not noticeably changed. When, however, the animal was required 

 to run, the abnormality became very apparent. Although he had recovered his 

 original weight within two months of the operation, he was unable afterwards 

 to run half a mile. The ability to do even a moderate amount of work had 

 therefore been lost, because the mechanism for increasing the heart action was 

 wanting. 



A. THE INHIBITORY NERVES 



If the vagus be cut in the neck and its peripheral end be stimulated with 

 tetanizing induction shocks, slowing of the heart beat or complete diastolic 

 standstill, according to the strength of stimulation, results. The vagus there- 

 fore inhibits the heart movements. We owe this important discovery to the 

 brothers, E. H. and E. F. Weber (1845). 



If both vagi of an animal be cut, the heart immediately beats faster. 

 Under normal circumstances therefore a constant restraint is being exercised 

 by the central nervous system upon the heart, in consequence of which it beats 

 more slowly than it otherwise would. 



The vagus influences not only the frequency of the heart beats, but also 

 the force. In fact it may happen under certain circumstances that when the 

 vagus is stimulated the pulse frequency remains entirely unchanged, while the 

 size of the contraction becomes constantly smaller ( Heidenhain, Gaskell). 

 According to Muskens, this takes place in the frog and turtle only in the 

 excised heart or after loss of blood. The heart relaxes in diastole more during 



