508 



PHYSIOLOGY 



an essential part of this action is inhibitory is shown by the effects 

 of exciting the motor area of the cortex after exhibition of strychnine 

 or during the local action of tetanus toxin. Whereas in the normal 

 animal closure of the jaw and extension of the fore limb are only 

 obtainable from one or two points on the surface of the brain, after 



the injection has taken place, 

 every part of the jaw area gives 

 closing of the jaw, every part of 

 the arm area gives extension of 

 the limb (cp. Fig. 174). 



Since the predominant in- 

 fluence of the motor cortex is 

 therefore inhibitory of the 

 stronger muscles of the body, as 

 well as of the tonus, which is con- 

 tinually and reflexly maintained, 

 it is not surprising that excision 

 of both hemispheres should give 

 rise to decerebrate rigidit)^ or that 

 destruction or division of the chief 

 direct tracts from the cortex to 

 the motor spinal mechanisms, viz. 

 the pyramidal tracts, should 

 determine increased tonus and 

 rigidity of the limbs the so-called 

 ' spastic ' condition observed in 

 cerebral paralyses. 



Two separable systems of motor 



FIG. 232. Diagram (from MOTT after MOST- i nner vatioil appear thus to control 

 AKOW) to show the interaction 01 the 

 different levels in the central nervous two sets of musculature. One SVS- 



system in the production of co-ordinated t exhibits the transient phases 



volitional movements. . . r 



s, sensory neuron ; B, bulb ; TII, thala- of heightened reaction which con- 



mus ; MA, motor area ; p, pyramidal gtitute reflex movements ; the 

 fibre ; c, cerebello - pontine nuclei ; 



vs, vestibular neuron (Deiters' nucleus), other maintains that steady tonic 



response which supplies the 



muscular tension necessary to attitude. Hughlings Jackson long- 

 ago called attention to this contrast between the two systems. He 

 pointed out that while the cerebrum innervates the muscles in the 

 order of their action from the most voluntary movements (the limbs) 

 to the most automatic (trunk), the cerebellum, or, as we should say 

 now, the whole proprioceptive system, innervates them in the opposite 

 order. The cerebellum therefore he regarded as the centre for con- 

 tinuous movements and the cerebrum for changing movements. 

 The increased tone of the paralysed muscles, observable after hem;- 



E 



