THE MECHANISM OF CO-ORDINATED MOVEMENTS 345 



The approach of a dog in answer to the calling of its name, the return of a, 

 when hungry to the place where it has been wont to receive food, such moven 

 may be taken as indicative of consciousness since they indicate the working of assoch t i 

 memory. Examined by this criterion all purely spinal reactions fail to evince features 

 of consciousness " (Sherrington). 



THE PART PLAYED BY AFFERENT IMPRESSIONS IN THE CO 

 ORDINATION OF MUSCULAR MOVEMENTS. Every reflex act is initiated 

 in the first place by some form of sensory stimulus. In the carrying out of 

 the muscular contractions and the resultant movements of the limbs, other 

 impulses are set up in the structures which subserve deep sensibility, in- 

 cluding those of muscles, which in their turn affect the excitability and the 

 activity of the motor neurons. These secondary afferent impulses are im- 

 portant whether the movements be aroused by immediate sensory stimulation 

 of the surface of the body, or through the higher parts of the brain, as in 

 volitional movements. 



Their significance is shown by the marked disorders of movement pro- 

 duced in a limb by section of some or all of its afferent nerves. Thus if 

 all the posterior roots supplying one hind limb of the frog be divided the 

 posture of the desensitised limb is abnormal, whether the frog be suspended 

 or be in a sitting posture. Such a frog generally swims with the desensitised 

 limb in permanent extension. The complete absence of muscular tone under 

 these circumstances has already been mentioned. When a contraction of 

 the quadriceps extensor is induced by a single shock applied to the intact 

 motor nerve, the curve obtained shows a relaxation line much slower and 

 more prolonged than when the cut nerve is similarly excited. In the latter 

 case, or when the posterior roots alone are divided, the lever at the end of re- 

 laxation dips below the base line with an inertia fling, which is never present 

 while the nerve is intact. The contraction of the muscle, when its afferent 

 path is intact, seems to develop reflexly in the muscle itself a condition of 

 tone which damps the inertia swing of the contraction. In the dog, after 

 section of the afferent nerves of one hind limb, this limb is not at first used 

 for walking ; it is kept more or less flexed at hip and knee, and later, when 

 it is employed in walking, it is lifted too high with each step. After division 

 of the afferent fibres of both limbs these appear as if they were affected 

 with motor paralysis. At first, during walking, the fore limbs simply drag 

 the hind limbs after them, though later, as the hind limbs are drawn along, 

 they make alternate movements and may ultimately afford a certain amount 

 of support to the body. 



Still more striking effects are observed in complete apaesthesia of the fore 

 limb in monkey or man. The limb is permanently paralysed ; it is never used 

 in climbing, or in the taking of food. That the peripheral motor mechanism 

 is intact is shown by the fact that stimulation of the appropriate area of the 

 cerebral cortex in such animals elicits at once a perfectly normal movement 

 of the hand or limb. It seems, however, impossible for the cortex to initiate 

 such movements in the absence of all afferent impulses arriving from the limb. 

 Similar paralysis was observed by Chas. Bell in the upper lip of the ass after 



