450 



PHYSIOLOGY 



the muscles of the trunk, and the extensor muscles of the limbs, stimulation 

 of the cortex produces opening of the mouth, flexion of the fore limb or of 

 the hind limb, more easily than any other movements. That 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 exten- 

 sion of the fore limb are only obtain- 

 able from one or two points on the 

 surface of the brain, after the injec- 

 tion has taken place, every part of 

 the jaw area gives closing of the jaw, 

 every part of the arm area gives ex- 

 tension of the limb (cp. Fig. 173). 



Since the predominant influence 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 de- 

 cerebrate rigidity, 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 cere- 

 bral paralyses. 



Two separable systems of motor 



system in the production of co-ordinated innervation appear thus to control two 



sets of musculature. One system exhi- 

 bits the transient phases of heightened 

 reaction which constitute reflex move- 

 ments ; the 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 continuous move- 

 ments and the cerebrum for changing movements. The increased tone of the 

 paralysed muscles, observable after hemiplegia, he ascribed to unbalanced 

 cerebellar influence. While there is no doubt that the cerebellum must play, 

 and does play, a considerable part in the production of decerebrate rigidity 



FIG. 234. Diagram (from MOTT after MON- 

 AKOW) to show the interaction of the 

 different levels in the central nervous 



' volitional ' movements. 



s, sensory neuron ; B, bulb ; TH, thala- 

 mus ; MA, motor area ; p, pyramidal fibre ; 

 c, cerebello-pontine nuclei ; vs, vesti- 

 bular neuron (Deiters' nucleus) 



