792 A MANUAL OF PHYSIOLOGY . 



(4) They may cross the middle line, after entering the cord, through 

 axons or collaterals (p. 770) which run in the anterior and also in 

 the posterior commissure, enter one of the ascending tracts on the 

 other side e.g., the tract of Gowers and continue without further 

 decussation up to their central destination. 



(5) They may spread from neuron to neuron in the tangle of the 

 grey matter itself, and pass out again at a different level into one of 

 the white tracts on the same or on the opposite side of the cord. 



Efferent Impulses from the brain may travel : 



(1) Through the direct or crossed pyramidal tract. 



(2) From one side of the cerebral cortex to the other, and then 

 down the pyramidal tracts corresponding to that side (?). 



(3) From the frontal part of the cerebral cortex, through the 

 anterior limb of the internal capsule to the grey matter in the pons, 

 and thence to the cerebellum by its middle peduncle. 



(4) From the occipital or temporal cortex, in the hinder rim of the 

 internal capsule, to the pontine grey matter and through the middle 

 peduncle to the cerebellum. From the cerebellum they may possibly 

 pass down to the nucleus of Deiters and thence along the antero- 

 lateral descending tract to the anterior horn of the cord, and 

 indirectly to the periphery. 



All the paths enumerated, as well as others to which it would 

 be tedious to formally refer, and which the ingenuity of the 

 reader may profitably be employed in constructing for himself, 

 from the data already given, are to be looked upon as possible 

 channels for the passage of impulses between the brain and the 

 periphery. But it must be distinctly pointed out that what is 

 certain is in this case much more limited than what is possible. 

 Among the efferent paths it is certain that the pyramidal tracts 

 are conductors of voluntary motor impulses, and that in most 

 individuals the great majority of such impulses decussate in 

 the medulla oblongata, only a small minority in the cord. For 

 a lesion involving the pyramidal tract above the decussation 

 of the pyramids causes paralysis of the opposite side of the 

 body, a lesion below the decussation paralysis of the same side. 

 It is certain that when one pyramidal tract has been destroyed, 

 in many animals at least, the resulting paralysis is soon recovered 

 from, at any rate to a great extent, and it is possible that in this 

 case the motor cortex on the side of the lesion has placed itself 

 again in communication with the paralyzed muscles through its 

 commissural connections with the opposite hemisphere. This, 

 however, is not the only alternative, for, as already pointed out, 

 the pyramidal tracts are not the only cortico-spinal paths which 

 can subserve volitional movements, and division of the anterior 

 portion of the antero-lateral column may cause deeper and more 

 permanent paralysis than division of the pyramidal tract. 



Decussation of the Sensory Paths. On the other hand, 

 it is certain that pathological or traumatic lesions, apparently 



