CUTANEOUS AND SOME OTHER SENSATIONS. 707 



ever, no anatomical guidance for such a crossing. The other path, along the 

 median posterior tract, comes to end in the gracile nucleus ; it has, indeed, 

 heen urged that the gracile nucleus is thus connected chiefly with the lower 

 limbs and lower part of the body, and that the analogous posterior root 

 fibres from the upper limbs and neck pass similarly into the cuneate nucleus, 

 or at least into the median division of that nucleus, but this cannot be con- 

 sidered as proved. Moreover, both the posterior columns, median and ex- 

 ternal, bring to these nuclei fibres which have started from some relay in the 

 gray matter lower down, and which are not fibres coming straight without 

 any relay from the posterior roots ; these, however, we cannot distinguish 

 from each other in their course beyond the nuclei. From the gracile and 

 cuneate nuclei the path onward is a double one, one broad, one narrow. The 

 broad path, the one having most fibres and presumably carrying most im- 

 pulses, leads to the cerebellum by the restiform body; and here the path, 

 previously continued exclusively along the same side of the cord, becomes 

 partly crossed though remaining partly uncrossed, the sensory decussation in 

 the bulb being the crossed and the other fibres passing from the nuclei 

 straight to the restiform body being the uncrossed one ( 525) ; the uncrossed 

 one we may, perhaps, look upon as really an upper part of the cerebellar 

 tract. The narrow path is the fillet ( 547), by which some of the fibres 

 from the nuclei are continued on toward the cerebrum. This path is a 

 crossed one, the crossing taking place in the sensory decussation, and it car- 

 ries relatively few impulses, the chief increase in the size of the fillet as it 

 passes onward being due to fibres coming from structures other than the 

 gracile and cuneate nuclei. 



Hence of the sensory impulses travelling along continuous tracts in the 

 spinal cord, these tracts apparently keeping always to the same side, the 

 great majority pass to the cerebellum ; and of these again the greater num- 

 ber, all those along the cerebellar tract, and some of those passing through 

 the gracile and cuneate nuclei, remain uncrossed to the end. The only path 

 by which all these impulses thus passing to the cerebellum can gain access 

 to the cortex of the cerebrum, is by some or other of the ties between the 

 cerebellum and the cerebral cortex. The relatively few impulses which pass 

 along the fillet are for the most part carried to the middle parts of the brain, 

 for only a small portion of the fillet passes to the cortex ( 547), and it is 

 not clear that this part of the fillet comes from the gracile and cuneate 

 nuclei, so that most of these impulses can gain access to the cortex only by 

 the relays of these middle parts of the brain. 



Very striking, indeed, are these constant relays along the path of sensory 

 impulses ; in this respect the sensory impulses offer a strong contrast to the 

 motor impulses. But a still more complex system of relays has to be men- 

 tioned ; for yet a third path is open for sensory afferent impulses along the 

 cord. We must admit the possibility of afferent impulses travelling along 

 the network of the gray matter, their path being either absolutely confined 

 to the gray matter, or leaving the gray matter at intervals, and joining it 

 again by means of those longer or shorter commissural or internuncial 

 fibres which unite the longitudinal segments of gray matter, and form no 

 inconsiderable portion of the whole white matter of the cord. We have 

 seen ( 499) that under abnormal circumstances impulses pass freely in all 

 directions along the gray matter, and we may conclude that under normal 

 circumstances they can pass along it under restrictions and along lines de- 

 termined by physiological conditions. The fibres in the white matter which 

 do not show either descending or ascending degeneration are probably, as we 

 have said ( 494), internuncial fibres, connecting segments of gray matter 

 in a longitudinal direction ; and, though we have no exact knowledge touch- 



