724 THE BRAIN. 



possess two hemispheres, and especially seeing that, as shown by speech, the 

 whole of each hemisphere is not identical in action with the whole of the 

 other, we may perhaps suppose that the fibres of the corpus callosum, which 

 form so large a part of the central white matter of the hemisphere, have 

 other duties than that of merely keeping the points of one hemisphere in 

 touch with the corresponding points of the other hemisphere. But, when 

 we have made every allowance for all these direct intercortical connections, 

 we are driven to the conclusion that the indirect ties between one part of 

 the cortex and another through the lower parts of the brain are of no less, 

 perhaps of greater importance. This, indeed, is shown by the relations of 

 the motor region. We have already urged, that even as regards the mere 

 carrying out of a skilled movement (and we may add, whether that be vol- 

 untary or involuntary in the ordinary common use of the words) the motor 

 region must have other ties with the part moved than merely the efferent 

 tie of the pyramidal fibres ; it must have sensory afferent ties, and the 

 course of these, including perhaps even those which belong to the muscular 

 sense, we may regard as an indirect one along the spinal cord and middle 

 parts of the brain, though the details are as yet unknown to us. It must, 

 moreover, as we have also seen, have ties, at least in many cases, with parts 

 other than the part moved, for instance with the general coordinating 

 machinery. And the ease with which some not very obvious change will 

 permit the stimulation of a limited motor area to start epileptiform convul- 

 sions shows how many and close are the ties in another direction. Further, 

 when we go beyond the final phases of the process in the motor cortex to 

 those which precede the issue of the efferent impulses, we find the ties mul- 

 tiplying. For instance, since our movements are so largely guided by visual 

 sensations, there must be ties between the motor cortex and the central 

 visual apparatus, it may be of the occipital cortex, but it may also be of 

 the lower visual centres. As we insisted, the motor area is only a link in a 

 complex chain ; and what we can see, dimly though it be, in reference to 

 the cortical motor processes, probably holds good for those other cortical 

 processes as well, of whose nervous genesis we know at present nothing. 

 Hence even the higher psychical events cannot truly be spoken of as func- 

 tions of the cortex, meaning that they are simply the outcome of molecular 

 changes in the cortical gray matter ; they are rather to be regarded as the 

 outcome of complex processes in which the parts of the brain below the 

 cortex play a part no less important than that of the cortex itself. If so, 

 the fibres passing down from the cortex to the middle brain have functions 

 by which they take part even in our psychical life, functions for which 

 neither the words motor nor sensory are fitting. 



ON THE TIME TAKEN UP BY CEREBRAL OPERATIONS. 



603. We have already seen ( 507) that a considerable time is taken 

 up in a purely reflex act, such as that of winking, though this is perhaps 

 the most rapid form of reflex movement. When the movement which is 

 executed in response to a stimulus involves cerebral operations a still longer 

 time is needed ; and the interval between the application of the stimulus 

 and the commencement of the muscular contraction varies according to the 

 nature of the mental labor involved. 



The simplest case is that in which a person makes a signal immediately 

 that he perceives a stimulus ex. gr., closes or opens a galvanic circuit the 

 moment that he feels an induction-shock applied to the skin, or sees a flash 

 of light, or hears a sound. By arrangements similar to those employed in 

 measuring the velocity of nervous impulses, the moment of the application 



