700 A MANUAL OF PHYSIOLOGY 



terior roots have the choice of many paths by which they 

 may reach the brain (Fig. 252). 



(1) They may pass directly up through thepostero-median column. 

 If they take this route, their course will be first interrupted by nerve- 

 cells in the gracile or cuneate nuclei in the medulla oblongata. 



(a) Thence they may find their way across the middle line by the 

 arcuate fibres of the upper or sensory decussation, and sweeping 

 along the fillet and the longitudinal fibres of the reticular formation 

 of medulla, pons and crus, and the sensory path in the hinder third 

 of the posterior limb of the internal capsule, finally arrive at the 

 cerebral cortex. Between the gracile and cuneate nuclei and the 

 cortex they may pass through nerve-cells in the optic thalamus and 

 the neighbouring region. 



(b) They may go from the gracile and cuneate nuclei to the resti- 

 form body of the same or the opposite side, and thence to the 

 cerebellum, in whose superior or middle peduncles they may cross 

 the middle line and proceed to the cerebral cortex. 



(2) They may pass up by the direct cerebellar tract and restiform 

 body to the superficial grey matter of the cerebellar worm. If they 

 take this route their course will be interrupted very soon after their 

 entrance into the cord in the cells of Clarke's column. Since the 

 superficial grey matter of the vermis is connected by association 

 fibres with the dentate nucleus, and the dentate nucleus by the 

 superior peduncle with the opposite cerebral hemisphere, this is 

 also a possible path to the great brain. 



(3) They may reach the antero-lateral ascending tract of the same 

 side through its cells of origin in the spinal grey matter, and passing 

 through the medulla and pons to the superior peduncle of the cere- 

 bellum, enter the grey matter of the worm. 



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

 axons or collaterals (p. 682) 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 Cowers 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, originating in 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 



