CONDITIONS AFFECTING THE MOTOR CENTRES. 



693 



the same number of stimuli is required to produce a continuous contraction, whether the cortical 

 centre, the motor nerve, or even the muscle itself be stimulated. With very feeble stimuli, 

 summation of stimuli takes place, for the muscular contraction only begins after several in- 

 effective stimuli have been applied. [It is generally held that the rhythm of a contracting 

 muscle is the same as the rhythm of the stimuli applied to its motor nerve, but Schafer and 

 Horsley contend that this holds good for rates of stimuli to about 10 or 12 per second. They 

 find that the same is true for the cortex cerebri, corona radiata, and medulla spinalis, viz. , that 

 the muscular response does not vary with the rhythm, i.e., number of stimuli per sec), but 

 that the rhythm is constant about 10 per sec. and independent of the number of stimuli per 

 sec, provided they are above 10 per sec. applied to these parts. Indeed, all voluntary contrac- 

 tions show a similar rate of undulation in the muscle-curve. Perhaps the rhythm of the 

 efferent impulses is modified in the motor nerve-cells of the spinal cord.] 



[The matter, as regards electrical stimulation of the cortex cerebri, resolves itself 

 into this, that stimulation of certain cortical areas always causes contraction in 



Si- 



Fig. 482. 



View of the brain from above (semi-diagrammatic). S end of ramus of the Sylvian fissure. 

 The other letters refer to the same parts as in fig. 479. 



definite muscles or groups of muscles, resulting in definite co-ordinated movements 

 on the opposite side of the body ; the areas have been called " motor areas." They 

 have been mapped out and ascertained in a large number of animals, and the 

 question comes to be Are there similar areas in manl] 



Primary Fissures and Convolutions of the Dog's Brain. The position of the motor centres in 

 the dog's brain is indicated in fig. 483, I and II. The dog's brain is marked by two "primary 

 fissures," viz., the sulcus cruciatus (S), which intersects the longitudinal fissure at a right 

 angle at the junction of its anterior with its middle third. This fissure has been called the 

 sulcus frontalis, or the fissura coronalis. The second primary fissure is the fossa Sylvii (F). 



