208 PRINCIPLES OF PHYSIOLOGY 



The olfactory bulbs and their roots are con- 

 nected with smell. In the middle region, on 

 each side of the great fissure known as the 

 Sylvian fissure, we find definite motor centres, 

 connected with the muscles and limbs and 

 with the muscles of the tongue and face on 

 the opposite side ; indeed it is highly probable 

 that every muscle of the body, even the laryn- 

 geal muscles associated with the production 

 of voice, has a centre in the cerebrum. 

 When these motor centres are irritated by 

 feeble electric shocks, movements of certain 

 muscles occur on the opposite side of the body. 

 Thus, stimulation of one centre, say on the 

 right side of the cerebrum, will cause a move- 

 ment of one of the muscles of the left hind leg. 

 Again, stimulation of another centre will 

 cause movement of the left fore leg. There 

 are complex centres for the lips, tongue, etc. 

 It must not be supposed, however, that these 

 motor centres are isolated or that they can 

 originate impulses. No doubt they are called 

 into action by nervous impulses of a sensory 

 character coming from other parts of the 

 cerebrum, or from below. Thus they also 



