588 SIGNIFICANCE OF THE NERVOUS SYSTEM 



by increasing the intensity of the stimulus, and (6) by heightening the 

 irritabihty of the nervous structures. The latter effect may be evoked 

 by any agent possessing a stimulating action upon the nervous system, 

 such as a weak solution of the sulphate of strychnin. If this drug is 

 injected under the skin covering the dorsal aspect of the frog, its grad- 

 ual absorption finally leads to an increased susceptibility to stimuli 

 which is clearly betrayed by the extensive and intense muscular spasms 

 resulting in consequence of even the slightest possible tactile or elec- 

 trical stimulus. The mere touch of the plate upon which the strych- 

 ninized frog has been placed or a current of air blown across the surface 

 of its body, now suffices to throw every muscle into a state of prolonged 

 contraction. The explanation usually given for this effect is that the 

 strychnin lessens the resistance to conduction. It is said to accomplish 

 this, end by increasing the continuity in the synapses, i.e., it is sup- 

 posed to bring the axon and dendritic terminals into closer relationship 

 so that the impulses are enabled to spread more readily from neuron to 

 neuron. 



Inhibition of Reflexes. — This phenomenon consists in a lessening 

 and final abolition of the motor response following the application 

 of a stimulus. It is commonly believed that this depression is brought 

 about by a blocking of the reflexes in their respective centers. The 

 impulses which accomplish this end are derived from different sources, 

 namely, (a) from the faculty of volition in the cerebral hemispheres, 

 (6) from higher reflex centers situated in the midbrain and hindbrain, 

 (c) from peripheral nerves, and (d) from a lessening of the irritability 

 of the nervous system as a whole. 



Cerebral Inhibition. — It is a matter of common experience that reflexes may be 

 suppressed by volitional efforts. While, under ordinary conditions, a touch upon 

 the cornea gives rise to a quick closure of the eyelids, special efforts may be made 

 to overcome this stimulus. In a similar manner, we may counteract the stimulus 

 which ordinarily gives rise to the act of coughing or sneezing. It seems, however, 

 that this cerebral inhibition necessitates two conditions, namely, that the excita- 

 tion be of moderate intensity, and that the reflex which we endeavor to suppress, 

 be in functional relation with volition. It must be evident that a strong excitation 

 must eventually overcome even very powerful counter efforts and furthermore, as 

 a large number of our reactions are not under the guidance of the will at any time, 

 it must be clear that volitional efforts cannot be brought to bear upon them. 

 This exception applies especially to the motor end-organs consisting of smooth 

 muscle tissue and situated in the domain of the sympathetic system. Thus, we 

 are quite unable to inhibit vasomotor and pilomotor reactions or to vary the size 

 of our pupils in antagonism to the stimulations received from the retinae. This 

 exception is also apparent in the case of several striated muscles, because we are 

 unable to influence the ci*emasteric reflex and to counteract the contraction of the 

 muse, bulbocavernosus. 



The inhibitory power of the cerebral cortex upon reflex action is well illustrated 

 by the changes in the "croaking reflex" of the frog occurring in consequence of the 

 removal of the hemispheres. Under normal conditions, this act necessitates a 

 certain psychic activity. ^ It is dependent upon certain elementary associations 



^ Goltz, Beitrag zur Lehre von den Funktionen der Nervenzentren des Frosches, 

 Berhn, 1863. 



