v SPINAL CORD AND NERVES 319 



said centres, due to the transient physiological processes known as 

 inhibition said facilitation (Bahnung*). 



The reflex actions of a spinal segment ^depend, not only on the 

 excitations that reach it by the respective afferent paths, but also 

 on influences from other portions of the nervous system. These 

 influences may be of such a character as to moderate or depress 

 its activity, or they may augment it. In the first case there is 

 inhibition, in the second facilitation, of the reflex. 



Inhibition, first discovered by the Webers in the action of the 

 excited vagus on the heart, was applied to the physiology of the 

 nervous system by Setschenow (1863). He observed that the frog 

 deprived of its whole brain developed stronger reflexes and reacted 

 to weaker stimuli. But if the cerebral hemispheres alone, without 

 the optic lobes and remainder of the brain, were extirpated, the 

 reflexes were not much affected. If, finally, the optic lobes of the 

 decerebrated frog were stimulated, e.g. with a crystal of salt, it was 

 seen to withdraw its foot from the acidulated water much later 

 than the normal frog. Setschenow concluded that the mesen- 

 cephalon is an inhibitory organ for spinal reflexes. 



Inhibitory reflexes were subsequently obtained from other 

 parts of the brain, and also from the cord itself, by direct or reflex 

 excitation. 



In the higher mammals, where the cord contains long cortico- 

 spinal paths, the brain has a marked inhibitory influence upon the 

 spinal reflexes, and these are facilitated by the removal of the 

 cortex or transection of the cor tico- spinal paths. Both in the dog 

 and in the ape this phenomenon is easily verified a short time 

 after the operation, i.e. when the spinal exaltation is due to the 

 onset of Wallerian degeneration, which acts as a continuous 

 irritant of the spinal tissue. That the brain can function as an 

 inhibitory organ for the spinal reflexes appears from the everyday 

 experience that we can sometimes voluntarily arrest, at other 

 times delay, more often modify certain reflexes, e.g. micturition, 

 defaecation, coughing, sneezing, etc. 



Another well-established fact is that excitation of one part 

 of the cord is able to inhibit the reflex activity of other parts. 

 This is seen particularly from the experiments of Goltz. If in the 

 spinal frog the sciatic nerve is stimulated electrically, no reflex is 

 evoked by applying acidulated paper to the skin. The arrest of 

 the frog's heart by rhythmically tapping the intestines does not 

 come off if the foot is pinched at the same time. The spinal snake 

 makes rhythmical pendulous movements which cease when its 

 body is lightly touched. Micturition already in progress can be 

 interrupted in a spinal dog by pinching the hind-foot or tail. In 

 the spinal cat suspended horizontally with relaxed limbs stimula- 

 tion of the skin of one foot causes drawing up of the homonymous 

 and extension of the contralateral limb; when both feet are 



