REFLEX PROCESSES 579 



The conclusion is, that when the cerebral cortex, or, more correctly, the nerve 

 cells of the spinal cord are active, stimulation of the cortex, under certain cir- 

 cumstances, has an inhibitory effect on such activity. 



The present status of our information on this subject may therefore be 

 summarized as follows : reflex processes may under certain circumstances be 

 retarded or entirely stopped by stimulation of different portions of the brain- 

 stem, of the cerebrum itself, or of afferent nerves in general. 



So much is fact, but whether the facts are to be explained by postulating 

 special inhibitory centers or in the manner conceived of by Goltz must remain 

 an open question. It would appear, however, as Goltz observes, to require an 

 absolutely overwhelming .number of inhibitory centers on the former hypothe- 

 sis, to account for all the reflex inhibitions with which we are acquainted. 



But, as Biedermann remarks, it is also possible to explain the phenomena 

 of inhibition in the central nervous system on the basis of special afferent 

 inhibitory nerves. If a center momentarily stimulated were to be acted upon 

 by a nerve of this kind its activity would be interrupted or diminished 

 i. e., would be inhibited. Biedermann points to the automatic regulation of 

 respiration (cf. page 328) and to certain locomotor reflexes (cf. page 587) 

 in the posterior extremities as examples of such inhibition. This inhibition 

 of nerve cells in the central system would be entirely analogous to those 

 inhibitions induced independently of the central system e. g., on the heart 

 by stimulation of the cut vagus or on the intestine by stimulation of the 

 cut splanchnic. 



D. AUGMENTATION OF REFLEXES 



But the effect of stimulating two intersecting pathways is not always an 

 inhibition; it may be an augmentation of the response. A stimulus in itself 

 subminimal applied to the motor cortex (rabbit) becomes effective if some 

 appropriate reflex stimulus, likewise subminimal, be applied at the same time. 

 This augmentation of the effect of one stimulus by the excitation of a different 

 pathway is seen when, after removal of the gray cortex, the corona radiata is 

 stimulated directly. The two excitations need not be simultaneous, the reen- 

 forcement occurs just the same if the second stimulus be applied some tenths 

 of a second after the end of the first. 



Exner, who has made a special study of this phenomenon, calls it facilita- 

 tion l (Bahnung) and ascribes to it very great significance in the functions of 

 the central nervous system. 



Exner regards the case of two central nuclei (e. g., the bilateral respiratory 

 center) so closely connected that excitation of one always or usually occurs 

 synchronously with excitation of the other, as a special form of facilitation. 

 When the two are connected by commissural fibers, charging the one produces 

 a change in the other which renders it more and more liable to discharge. 



Just as self -culture often amounts to the suppression of unpleasant or unde- 

 sirable reflexes, so its aim often is to establish reflexes of a pleasing or desirable 

 character, and in this the reenforcement of one nervous pathway by another is of 

 great service. 



1 Sherrington's term. 



