562 THE SPINAL CORD. 



cord from one part to another. It may be that some of the finer medullated 

 fibres do really enter into the groundwork, and so contribute to the nervous 

 web ; but our knowledge is too imperfect to afford a clear decision on this 

 point. Our inability to define its exact limits need not, however, prevent 

 our recognizing the existence of the groundwork. 



The prominence in this groundwork of the larger nerve-cells has led to 

 the conception that the powers of the spinal segment are exercised by these 

 nerve-cells to the exclusion of the other elements of the nervous web. But 

 such a view has not been adequately proved. What we do know is that the 

 nuclei and cell-bodies of the cells of the anterior horn exercise an important 

 influence on the nutrition of the fibres of the anterior root which proceed 

 from them, and possibly also influence the nutrition of the other branches 

 of the cells forming part of the groundwork ; and these cells are probably 

 so conspicuous a feature of every section of the spinal cord because of the 

 important task intrusted to them of maintaining in due order the nutrition 

 of the long stretch of motor fibres reaching from them to the muscular 

 fibres or other peripheral organs. The fibres of the posterior root are not so 

 obviously connected with the conspicuous cells of the gray matter ; indeed, 

 as we have said, it may be doubted, though the view is maintained by some, 

 whether any cell intervenes to secure the continuity of a posterior fibre with 

 the groundwork, a division of the axis-cylinder serving this purpose ; and 

 this becomes intelligible when we bear in mind that the posterior fibres are 

 governed as far as their nutrition is concerned by the nerve-cells of the gan- 

 glion on the posterior root, which ought probably to be considered as much 

 a part of the spinal cord as the cells of the anterior horn. The nerve-cell 

 of the ganglion is adequate to secure the due nutrition of the nerve-fibre 

 until it joins the groundwork, and probably helps to maintain the nutrition 

 of the groundwork itself. 



Hence we may perhaps, until fresh evidence to the contrary is brought 

 forward, incline to the view that the powers of the gray matter do not 

 depend on the conspicuous cells alone or even chiefly, but on the peculiar 

 molecular constitution and nature of the whole groundwork. The nuclei of 

 the cells of the anterior horn with the cell substance adjacent to each and 

 the cells of the ganglia on the posterior root probably govern the nutrition, 

 and so the functional activity of the groundwork as well as of the issuing 

 and entering fibres ; but there appears to be as yet no convincing evidence 

 of any other peculiar powers confined to the cells and absent from other 

 parts of the groundwork. We may add that, in accordance with this view 7 , 

 the other cells of the gray matter, such as those of the vesicular cylinder, 

 are to be regarded as of importance for governing the nutrition of fibres, 

 commissural and others, starting from the spinal segment, and of the part 

 of the groundwork from which by their mediation the fibres start, rather 

 than for determining the functions of the groundwork of the segment or of 

 the fibres receiving impulses from it. 



493. The segmental groundwork of gray matter belonging to each pair 

 of spinal nerves is so fused with that of all the other pairs as to form along 

 the whole length of the cord a mass of gray matter which appears, under 

 certain circumstances at all events, to be continuous in the sense that impulses 

 may pass in all directions along it. But each spinal segment is in addition 

 connected by means of tracts of white matter with parts more or less distant. 

 The crossed pyramidal tract is such a longitudinal commissural tract, con- 

 necting apparently each spinal segment in succession with a certain part of 

 the cortex of the cerebrum. We have reason to think, as we shall see later 

 on, that impulses descending this or that fibre or group of fibres of this tract 

 give rise to the issue of motor impulses along this or that fibre or group of 



