390 PHYSIOLOGY 



secondary afferent impulses are important 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 

 produced 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 active 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 relaxation 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 para- 

 lysed ; 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 section of the corresponding branches of both fifth 

 nerves, and was interpreted by him as indicating a possible motor 

 function for these nerves. 



In these phenomena of sensory paralysis we are dealing with the 

 effects produced by the deprivation of two distinct classes of afferent 

 impressions, viz. those from the skin, and those from the deep struc- 

 tures and muscles. The phenomena due to these two factors may be 



