838 THE CENTRAL NERVOUS SYSTEM 



with the usefulness of the limb, the condition being called apesthesia. 

 The exact results depend somewhat on the type of animal. If all the 

 posterior roots of the anterior extremity are cut in a monkey, the 

 corresponding limb will not be used in climbing or for other purposes. 

 It will appear to be completely paralyzed, unless when the opposite 

 normal limb is in vigorous activity, when the apesthetic arm may be 

 moved in association. 



On careful examination, however, it will be found that marked dif- 

 ferences exist in the types of paralysis produced by the section of the 

 anterior and the posterior roots. When a motor root is cut no reflexes 

 are possible either from the skin or from the cerebral cortex, and the 

 muscles undergo atrophy. After section of the posterior root, on the 

 other hand, although reflexes from the skin area affected are impos- 

 sible, yet movements may be elicited by artificial stimulation of the 

 cerebral cortex, and the muscles do not atrophy to the same extent. 



If only one sensory root of an extremity is left uncut for example, 

 the last cervical so that the skin of the hand is still supplied with 

 sensation but all the deep receptors are severed, then the limb may be 

 used to a modified degree. It may be used by the monkey to pick up 

 nuts, but the movement will be distinctly clumsy and ataxic in nature. 

 Instead of neatly picking up the nuts, he will 'make wild movements 

 and often miss them. 



The apesthesia is not so profound in lower animals. After section of 

 all the sensory roots to both hind limbs in the dog, there may be a 

 certain attempt at walking on the part of the affected limb; that is to 

 say, when the animal tries to progress, the 'hind limbs, although at first 

 merely dragged along the ground, afterwards begin to execute walking 

 movements, which however are very jerky or ataxic in nature and con- 

 tribute little to the forward progression of the animal, although he 

 may succeed to a certain extent in supporting the body by the hind limbs. 



The importance of the sensory root in controlling the contraction of 

 the muscles is further illustrated by comparing the contraction curve of 

 a muscle produced by stimulating its uncut motor nerve with that pro- 

 duced by stimulating the peripheral end of the cut nerve. In the former 

 case, the curve is more prolonged and shows a gradual relaxation, 

 whereas when the peripheral end of the cut nerve is stimulated, the con- 

 traction is brief and the relaxation is followed by a distinct rebound 

 or "inertia swing," as it is called. That this difference depends on 

 afferent impulses is indicated by the fact that, after section of the 

 posterior roots, stimulation of the uncut nerve in the limb will produce 

 the same effect as occurs when the cut nerve is stimulated. These re- 

 sults can be very clearly obtained in the case of the frog, in which 



