CHAP. IT.] 



THE BRAIN. 



753 



just described. When the movements are brought about by 

 stimulation of the fibres in some part of their course, in the in- 

 ternal capsule for instance, there can be no doubt that the stimu- 

 lation starts impulses which, travelling down the tract to the 

 origins of certain cranial or spinal nerves, in some way give rise 

 to coordinate motor impulses along the motor fibres of the nerves ; 

 and we may with reason speak of the impulses then passing along 

 the tract as motor or efferent in nature. When the stimulus is 

 applied direct to the cortex, we may assume that processes, 

 started in the grey matter, eventuate in similar efferent im- 

 pulses along the fibres of the tract. All the evidence leads us 

 to regard this tract as an efferent tract. 



When the spinal cord is divided in the lower dorsal region 

 and the electrodes of an electrometer are brought into connec- 



FIG. 128. THROUGH THE Cnus AND ANTERIOR CORPORA QUADRIGEMINA. 



(One half only is shewn.) (Sherrington.) 



(In the line 114, Fig. 108.) 



Py. the pyramidal portion of the pes. FT. the region of the pes occupied by 

 fibres from the frontal portion of the cortex. Pr. O. the region occupied by 

 fibres coming from the occipital portion of the cortex, y. fibres coming from the 

 fillet. Op. the optic tract. F. the fillet, L the lateral portion, m. the median 

 portion. I. the posterior longitudinal bundle. B. a. the brachium of the anterior 

 corpus quadrigeminum. x. fibres from the posterior commissure of the cerebrum. 

 r. raphe. S. n. substantia nigra. JR. n. red nucleus. C. g. 1. lateral, and C. g. m. 

 median corpus geniculatum. Pvr. pulvinar of optic thalamus. A. Q. n. nucleus 

 or grey matter of anterior corpus quadrigeminum. III. n. nucleus of III. third 

 nerve. Ill', rootlets from the dorsal part of III. n. the nucleus of the third 

 nerve which cross the median line to emerge with rootlets derived from the 

 nucleus of the opposite side. s. m. superficial layer of fibres of the ant. corp. 

 quad. d. m. deep layer. V. d. descending root of the fifth nerve. Aq. aqueduct 

 surrounded by cerebral grey matter. 



tion with the transverse cut surface and with some point of the 

 longitudinal surface above, the electrometer gives evidence of 

 currents of action (manifested as negative variations of a demar- 



48 



