306 PHYSIOLOGY 



increasing height of contraction, which depends on the state of the contracting 

 tissue itself and cannot be evoked by changes in the strength of the stimulus. 

 This process of facilitation or ' Bahnung ' is of great interest in connection 

 with the development of ' long paths ' in the central nervous system, and 

 more especially with the acquirement of new reactions by the higher animals. 

 The Law of Facilitation is really the Law of Habit. When an impulse has 

 passed once through a certain set of neurons to the exclusion of others it 

 will tend, other things being equal, to take the same course on a future 

 occasion, and each time that it traverses this path the resistances in the 

 path will be smaller. Education is the laying down of nerve-channels in 

 the central nervous system, while still plastic, by this process of ' Bahnung ' 

 along fit paths, combined with inhibition (by pain) in the other unfit paths. 

 Memory itself has the process of ' facilitation ' as its neural basis. 



(7) INHIBITION. The constant occurrence of a reaction in response 

 to a given stimulus is obtained only if care be taken to isolate the segment 

 of the central nervous system involved from the entry of other afferent 

 stimuli. As a rule, if two stimuli be applied simultaneously at different 

 points, the reaction which ensues will not be a combined one, the resultant 

 of the reactions which would be normally conditioned by each single stimulus, 

 but will be a response to one of the stimuli, which we must therefore regard 

 as the more effective. The reaction to the other stimulus is either abolished 

 altogether or comes on after a considerable period of delay. The central 

 nervous system can apparently attend to only one thing at a time. In 

 physiological terms we should say that every effective reaction inhibits 

 every other reaction. In the spinal cord of the frog the normal withdrawal 

 of the foot in response to stimulation of the toe of the same side can be 

 inhibited by strong stimulation of the other sciatic nerve, by stimulation 

 of the spinal cord at a higher level, or by stimulation of the optic lobes. 

 Immediately after pithing the brain of the frog, the whole animal becomes 

 flaccid and motionless, and for the next few minutes it is impossible to elicit 

 any reaction by stimulation, however strong, applied to the skin of the 

 body. In the production of this condition of ' shock ' the inhibition of all 

 the spinal centres, produced by the strong stimulation of the injury to the 

 brain and medulla, plays at any rate an important part. /We may say that 

 the passage of an impulse through a chain of neurons diminishes the block 

 for subsequent impulses at each synapse that it traverses, but increases 

 during its passage the block in all the adjacent synapses J 



In dealing with the special reactions of the spinal cord we shall have 

 occasion to refer more fully and in greater detail to many of these properties 

 which are characteristic of all reflexes. Before, however, treating of the 

 functions of the separate parts of the central nervous system in the higher 

 mammals, it may be of interest to consider the exact nature of the structure 

 intervening between neuron and neuron at each field of conjunction or 

 synapse, as well as the significance of the two chief elements of the central 

 nervous system, nerve-cell and nerve fibre, in the production of co-ordinated 

 purposive reactions. 



