v SPINAL CORD AND NERVES 321 



The inhibition due to direct or indirect excitation of a spinal 

 centre is usually quite transient in decerebrated animals. When 

 the stimulation is sufficiently prolonged, the inhibition is followed 

 by a functional rise, which accords well with the alternating clonic 

 character of the muscular reactions in decerebrated animals. 



Transection of the cord also induces inhibition in its caudal 

 segments, which is more pronounced in the segments nearest the 

 section and gradually declines in the more remote segments. This 

 is plain from Rosen thal's experiments (1873). He showed that 

 the latent time of the reflex evoked in the frog's hind-limb by 

 stimulating the skin of the opposite limb is longer in low than 

 in high transection. Bickel at a later date (1898) proved on the 

 frog, salamander, and tortoise that the reaction time is longer 

 when the cord is cut below the brachial plexus than when it is 

 divided immediately below the medulla oblongata. Further, De 

 Boeck (1887) found that a stronger stimulus was needed to excite 

 a reflex in the rabbit when the cord was divided than when the 

 section lay above the spinal bulb. 



The opposite effect, facilitation or augmentation (Bahnung) 

 of the spinal reflexes, was first pointed out by Exner in 1882. 

 He saw in the rabbit that on simultaneously stimulating the 

 cortical centre for a given muscle of the leg and a point on the 

 skin of the leg by which the same muscle was excited, the reflex 

 contraction was more energetic than when the cortex alone, or 

 the skin alone, was excited. On reducing the strength of the 

 cutaneous stimulus till it became subliminal, it was made efficient 

 again when a cortical stimulus was applied two seconds pre- 

 viously. Adequate skin stimuli similarly rendered subliminal 

 cortical stimuli effective. When both stimuli taken singly were 

 subliminal, each made the other efficient if the interval between 

 them did not exceed one-eighth of a second. 



Sherrington gives other instances of reflex facilitation. He 

 states that the reflex excited from an afferent root of a spinal 

 animal by a given minimal stimulus can be evoked by a weaker 

 stimulus when other adjacent roots are previously excited. 



In 1905 he analysed the fundamental characteristics of a 

 specific reflex in the dog, the "scratch reflex." On applying 

 certain stimuli within a wide saddle-shaped zone of the skin on 

 the back and flanks (Fig. 185) of a dog, after high thoracic tran- 

 section, the hind-leg on the same side executes a scratching move- 

 ment. This movement is produced by flexion of the hip, knee, 

 and ankle, which is rhythmically repeated about four times a 

 second. The sensory nerve-endings which discharge the reflex 

 (the " receptors ") lie on the surface of the skin and seem to be in 

 close relation with the hair follicles. The reflex can be evoked by 

 mechanical stimuli rubbing the skin or lightly pulling the hair- 

 as well as by electrical excitation weak faradic currents, constant 



VOL. Ill Y 



