654 COURSE OF THE SENSORY NERVES. 



becomes connected with a multipolar nerve-cell in the anterior horn of the grey 

 matter of the spinal cord, so that they have the longest course of any fibres in the 

 central nervous system.] 



Variation in Decussation. There are variations as to the number of fibres which cross at 

 the pyramids {Flcchsig). In some cases the usual arrangement is reversed, and in some rare 

 instances there is no decussation, so that the pyramidal tracts from the brain remain on the 

 same side. In this way we may explain the very rare cases where paralysis of the voluntary 

 movements takes place on the same side as the lesion of the cerebrum (Morgagni, Pierret). 

 This is direct paralysis. [Usually about 90 per cent, of the fibres decussate.] 



The motor cranial nerves have the centres through which they are excited 

 voluntarily in the cortex cerebri ( 378). The paths for such voluntary impulses 

 also pass through the internal capsule and the crusta of the cerebral peduncle. 

 [In the internal capsule, the fibres for the face (and tongue) lie in the knee, while 

 they occupy the part of the middle of the crusta next the middle line. Their 

 course is then directed across the middle line to their respective nuclei, from which 

 fibres proceed to the muscles supplied by these nuclei.] The exact course of many 

 of the fibres is still unknown. The hypoglossal nerve runs with the pyramidal 

 tracts, and behaves like the anterior root of a spinal nerve ( 354, 357). 



[Sensory Paths. Our knowledge is by no means precise. Sensory impulses, 

 passing into the cord, enter it by the posterior nerve-roots, and may pass to the 

 cerebrum or cerebellum. If to the cerebellum, the course, probably, is partly 

 to the direct cerebellar tract and posterior column to the restiform body, thence to 

 the cerebellum. If to the cerebrum, they cross the middle line in the cord not 

 far above where they enter and pass to the lateral column, in front of the pyramidal 

 tract. Some enter the posterior column, and others ascend in the grey matter to 

 pass upwards. As the two subdivisions of the posterior column terminate above 

 in the nuclei of the funiculus gracilis and funiculus cuneatus, and this column 

 contains fibres from the posterior root, it is suggested that above the clava and 

 cuneate nucleus the fibres cross in the superior pyramidal decussation to reach the 

 pons and tegmentum. In the medulla, it is probable that those fibres which do not 

 decussate there do so in the pons, the impulses perhaps travelling upwards in the 

 formatio reticularis, thence, into the posterior half of the pons, into the tegmentum 

 of the crus under the corpora quadrigemina, to enter the posterior third of the 

 posterior limb of the internal capsule (fig. 500, S). But, of course, the sensory 

 fibres from the face have to be connected with the sensory centres in the cerebrum, 

 so that the sensory paths from the cord, i.e., from the trunk and limbs, are joined 

 by those from the face in the pons, and they also occupy part of the posterior third 

 of the posterior segment of the internal capsule, so that this important part of the 

 internal capsule conducts sensory impulses from the opposite half of the body. 

 Some of the fibres pass into the optic thalamus, and others enter the white matter 

 of the cerebrum, but their exact course is very uncertain. The sensory fibres 

 derived from the organs of special sense, e.g., the ear, go to the superior temporo- 

 sphenoidal convolution, but whether directly or indirectly we do not know ; perhaps 

 some of those for vision traverse the optic thalamus. Some of the afferent fibres 

 perhaps go to the occipital region, and Gowers asserts that some of them go to the 

 parietal and central regions, i.e., to the "motor" regions, for he holds "that disease 

 of the motor cortex often causes impairment of the tactile sensibility."] 



[Charcot has called the posterior third of the posterior segment of the internal 

 capsule, lying between the posterior part of the lenticular nucleus and the optic 

 thalamus, the "carefour sensitiv" or "sensory crossway " (fig. 500, S). If it 

 be divided there is hemi-anaesthesia of the opposite side.] 



Sensory Decussation in Cord. As the greater part of the sensory fibres 

 from the skin decussate in the spinal cord, and thus pass to the opposite side of the 

 cord (fig. 463), unilateral section of the spinal cord in man (and monkey Ferrier) 



