450 



PHYSIOLOGY 



bral hemispheres be removed, as by a section through the crura cen-lni 

 or in front of the mid-brain, this postural tonus is increased and the animal 

 enters into the condition of ' decerebrate rigidity/ Dest met ion o! one laby- 

 rinth diminishes the tone on the same side of the body ; section of all the 

 afferent nerves from a limb abolishes the tone in that limb, so that its posture 

 thereafter depends entirely on gravity. 



The movements which are excited in such animals by cutaneous stimula- 

 tion involve as a necessary factor inhibition of the postural tone as well as 



co-operative inhibition of the an- 

 tagonistic muscles. In the same 

 way excitation of the motor area of 

 the cortex has as its most essential 

 feature an inhibitory action on the 

 postural tonus in addition to its exci- 

 tatory action on the muscles con- 

 cerned in the movement. A cer- 

 tain antagonism is evident between 

 the total action of the cerebral hemi- 

 spheres and that of the propriocep- 

 tive part of the central nervous, sys- 

 tem. Whereas in the decerebrate 

 animal there is increased tonus in 

 the masseters, in the neck muscles. 

 the muscles of the trunk, and the 

 extensor muscles of the limbs, stim- 

 ulation 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 cor- 

 tex 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 obtainable 

 only 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 (ry>. Fig. 173). 



Si nee 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 decerebrate rigidity, or that destruction 

 or division of the chief direct tracts from the cortex to the motor spinal 



FIG. 234. Diagram (from MOTT after MON- 

 AKOW) to show the interaction of the 

 different levels in the central nervous 

 system in the production of co-ordinated 



' volitional ' movements. 



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

 mil- : M \. motor area ; p, pyramidal fibre ; 



.. rereliellu-pniitiiic nuclei; vs, vesti- 

 liuhir neuron (Deiters" 



