578 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



and gesticulations belong to the same class. (Recall, e. g., the old story of 

 Demosthenes. ) 



The suppression of reflexes may be explained as follows : When the cerebrum 

 is removed from an animal it is observed that complex systemic reflexes can be 

 more readily and more regularly induced than before. The cerebrum has there- 

 fore the power either consciously or unconsciously to restrain reflexes, which are 

 discharged more easily by lower centers working alone. It may be supposed to 

 have the same power over acts which have come to be easily performed because 

 certain pathways are strongly developed either by inheritance or as the result 

 of habit. Exercise of this control, however small it may be at first, will accom- 

 plish the suppression of such processes as secretion of tears and the like. 



But it would be incorrect to suppose that reflexes are inhibited only through 

 the cerebrum. It is not a difficult matter to demonstrate such inhibitions on 

 animals devoid of the cerebrum, notwithstanding that excitability of the reflea 

 arc is greatly increased by the operation. On the strength of experiments involv- 

 ing chemical stimulation of the frog's skin, Setschenow taught that special 

 inhibitory centers are to be found in the neighborhood of the optic thalami, the 

 corpora quadrigemina and the anterior portion of the medulla oblongata, under 

 the influence of which reflexes from the spinal cord are retarded. These centers 

 can be excited directly or by stimulation of afferent nerves, he said, and are 

 always active throughout life. On the other hand, there are no inhibitory 

 mechanisms for reflexes aroused by tactile stimuli. 



Goltz demonstrated, however, that reflex acts induced by all sorts of tactile 

 stimuli, as mere contact, stroking the skin, etc., can under certain circum- 

 stances be entirely suppressed, and he laid it down as a general rule that 

 any center mediating a definite reflex suffers a distinct loss in excitability 

 whenever it is acted upon at the same time by any other pathway not con- 

 cerned in that particular reflex. 



Goltz instanced the following examples of inhibitions of this sort. (1) The 

 heart may be brought to a complete standstill by lightly tapping the abdominal 

 viscera (Klopfversuch). But this otherwise invariable result is not obtained if 

 at the same time a sensory nerve of one of the legs is stimulated powerfully. 

 (2) If the skin between the fore legs of a male frog taken in the breeding season 

 be stimulated lightly with the finger after the animal has been beheaded, the 

 finger will be clasped firmly by the fore legs (clasping reflex). Ordinarily this 

 reflex never fails, but if a drop of acetic acid be applied to the animal at the 

 same time it very often fails. (3) If a strong solution of common salt be 

 injected under the dorsal skin of a frog, all reflexes cease. The limbs hang 

 perfectly limp and are not drawn up even when vigorously scraped with a knife. 

 This condition lasts for some minutes and then the reflexes gradually return 

 (Bethe). 



Heidenhain and Bubnoff's observations on morphinized dogs furnish us fur- 

 ther examples of inhibitory processes in the central nervous system. It is well 

 known that muscular contractions may be induced by artificial stimulation of 

 certain areas of the cerebral cortex (of which more in Chapter XXIV). Such 

 contractions in morphinized dogs are long drawn out, disappearing but gradu- 

 ally when the stimulation ceases. But if, while the after-effect is still on, the 

 skin be stroked lightly or some other sensory stimulus be applied, the contracted 

 muscle immediately relaxes; moreover, the same result is obtained if the same 

 area of the cortex be given another, this time a weak or transitory stimulus. 



