CARDIAC INHIBITION AND ACCELERATION 



323 



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the result that the pressure now falls somewhat below its normal level. 

 Subsequent to this point normal conditions are again established. 



As might be expected, these compensatory changes are not always 

 of the same intensity, because the irritability of the vasomotor mech- 

 anism differs in the same degree as that of the entire nervous system. 

 It is obvious, however, that a close reflex correlation exists between the 

 cardiac and vasomotor centers, so that a reduction in the ventricular 

 output may be compensated for immediately by a constriction of 

 the blood-vessels. This is of greatest importance, because the functions 

 of the different colonies of cells in our body 

 must necessarily cease, if the pressure under 

 which they obtain their nutritive material 

 falls below a certain minimal value. For 

 this reason, even a relatively brief inhibi- 

 tion of the heart must be associated with 

 a general depression of function which 

 makes itself felt most strikingly by a loss of 

 our psychic activities. If continued for an 

 undue length of time, the inhibition must 

 necessarily be followed by certain disturb- 

 ances of function which are not so easily 

 compensated for and remedied. The '^ es- 

 cape of inhibition" may be said to consti- 

 tute a safety device of the body to prevent 

 fatal consequences from this source. 



The Character and Nature of the Ac- 

 celeration. — The action of the accelerator 

 fibers may be tested experimentally in 

 mammals as well as in lower forms. In 

 the former, these fibers may be isolated dis- 

 tally to the thoracic sympathetic ganglia, 

 while in the frog and aUied animals, they 

 may be rendered accessible directly beside 

 the vertebral column. As is indicated in 

 Fig. 171, the latter eventually unite with 

 those of the vagus and finally terminate 

 in the heart. The cardiomotor fibers, therefore, may be reached in 

 this animal in three different places. Their stimulation at A, where 

 the vagus alone is affected, results in an inhibition, while the stimula- 

 tion of the sympathetic chain at B gives acceleration. For reasons 

 discussed previously, the excitation of the vagosympathetic at C is 

 followed by an inhibition. 



The accelerators produce their effect after a considerable latent 

 period, but when once established, the acceleration continues as a rule 

 for some moments after the cessation of the excitation. Ten or 

 twenty seconds- frequently elapse before a marked increase in the 

 cardiac rhythm is observed, while, in the case of the vagus, the latent 



Fig. 171. — Schema to 

 Show the Course of the Car- 

 diac Nerves in the Frog. 



A, vagal fibers are still 

 separate; B, sympathetic fibers 

 are still separate; C, both 

 types of fibers have combined 

 to form the vagosympathetic 

 nerve. 2, Remak's ganglion; 

 B, Bidder's ganglion. 



