56 ESSENTIALS OF PHYSIOLOGY. 



In man, the motor impulses are conveyed from the brain to the 

 neurons of the different anterior nerve roots almost entirely by the 

 pyramidal (cerebro-spinal) tracts, but in the lower animals other tracts 

 are also used. As has already been stated, the greater part of the 

 motor path from the brain crosses in the medulla oblongata to form 

 the crossed pyramidal tract, and the fibres of this tract terminate in 

 the grey matter of the spinal cord in relation with the neurons con- 

 nected with the anterior nerve roots of the same side. Hence a lesion 

 involving one of these tracts results in motor paralysis on the same 

 side of the body. There will be a certain degree of weakness of muscles 

 on the opposite side of the body after a unilateral lesion of the human 

 spinal cord, because of the interference with the direct pyramidal tract, 

 the fibres of which cross immediately before their termination ; but 

 this is relatively insignificant, and, as the fibres of the direct pyramidal 

 tract have all crossed in the cervical and thoracic regions, a unilateral 

 lesion in the lower thoracic region will result in motor paralysis of the 

 homolateral hind limb only. 



The localisation of the paths for the various sensory impulses (fig. 11) 

 has been ascertained by the study of diseased conditions in man, as well 

 as by observing the results of experimental localised lesions in animals. 

 By such methods it has been shown that the funiculus gracilis and the 

 funiculus cuneatus are concerned with the transmission of those 

 kinsesthetic (muscle sense) impulses which pass to the cerebral hemi- 

 spheres and cerebellum, the paths crossing in the medulla oblongata 

 just above the decussation of the motor tracts ; a cell station occurs on 

 these paths below their decussation. 



The direct cerebellar and the antero-lateral ascending (Gowers') 

 tracts also convey kinsesthetic impulses, but these differ from those 

 conveyed to the cerebral hemispheres in that they are not connected 

 with conscious sensation. Both these tracts are uncrossed in their 

 course to the cerebellum, and in neither is there a cell station on 

 the path. 



The fact that the four long ascending tracts convey impulses of 

 muscle sense is to be associated with the importance of the function of 

 equilibration, a function which requires for its performance delicate 

 muscular adjustments. 



Impulses of pain, heat, and cold are conveyed into the posterior 

 horn of grey matter by fibres of the posterior roots, They then pass 

 by a second neuron to the spino-thalamic fibres of the opposite side, 

 and thus reach a cell station in the optic thalamus, from which they 

 are passed on to the cerebral cortex. Tactile impulses pass up the 

 posterior column of the same side for four or five segments before 



