FUNCTIONS OF THE BRAIN. 829 



sensory impulses travel through the reticular formation, through the 

 posterior half of the pons, and enter the tegmentum of the crura cerebri, 

 pass under the corpora quadrigemina to enter the posterior third of the 

 posterior limb of the internal capsule, and thence radiate to the cortex 

 of the occipital and temporo-sphenoidal lobe. The path of the sensory 

 fibres, therefore, in some part of its course undergoes decussation, either 

 in the cord, the medulla, or the pons, so that the cortex of each cerebral 

 hemisphere receives sensory impulses which originate in impressions 

 made on the opposite side of the body : hence, a destructive lesion of the 

 cerebral cortex, internal capsule (posterior third), or of one-half of the 

 cord causes anaesthesia of the opposite side of the body. The major part 

 of the crossing, however, occurs in the posterior commissure of the cord. 

 Voluntas motor impulses originate in the cells of the cortex in the 

 motor areas of the cerebrum, pass through the radiating fibres of the white 

 matter of the cerebral hemispheres, to converge into the internal capsule, 

 which is a collection of white nerve-fibres lying between the caudate 

 nucleus and optic thalamus internally and the lenticular nucleus externally. 

 They then enter the basis of the crura cerebri, occupying its middle third, 

 the fibres for the face being next the middle line and those for the leg most 

 external, the fibres for the arm lying between the two ; they then pass to 

 the pons, the facial fibres here undergoing decussation (becoming con- 

 nected with the nuclei of the facial and hypoglossal nerves), the others 

 continuing on the same side to the anterior pyramids of the medulla 

 oblongata, where the major part crosses to the opposite side of the cord, 

 where they descend in the lateral column (the crossed pyramidal tract) ; 

 while the uncrossed fibres descend in the anterior columns of the same 

 side, ultimately, in all probability, crossing through the white commissure. 

 All the fibres of both pj^ramidal tracts terminate at different levels in the 

 multipolar cells of the anterior cornua of the gray matter of the cord, 

 and from each of these cells originates a single unbranched process which, 

 joining with similar fibres, passes out of the cord in "the anterior roots of 

 the spinal nerves. The course of these motor and sensory fibres is like- 

 wise shown in Figs. 360 and 361. The cerebellum receives through its 

 inferior peduncle the afferent fibres derived from the lateral (direct cere- 

 bellar tract) and posterior columns of the cord, as well as from the gray 

 matter. The muscular sense is supposed to be conducted by means of 

 Clarke's column, together with the direct cerebellar columns, while tactile 

 sensations pass through Burdach's column. While, therefore, the sensa- 

 tions of pain are conducted directly to the cerebrum, tactile and muscular 

 sensations first reach the cerebellum and may from thence be conducted 

 to the cerebrum through the superior peduncle of the cerebellum, passing 

 into the posterior part of the corpora quadrigemina and then, perhaps, 

 forming connection with the caudate nucleus. 



