432 THE NERVOUS SYSTEM. 
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different stages of its development, it is a comparatively easy matter to locate the different tracts 
of fibres by evidence of this kind. Speaking broadly, the tracts which myelinate first are those 
which bring the cord into relation with the peripheral parts (skin, muscles, etc.) ; then those 
fibres which bind the various segments of the cord together ; next, those which connect the cord 
with the cerebellum ; and, lastly, the tracts which connect the cord with the cerebrum. The 
nervous apparatus for the performance of automatic movements is fully provided, therefore, before 
this is put under the control and direction of the higher centres. It by no means follows that in 
all the higher animals corresponding strands myelinate at relatively corresponding periods. Take 
the case of a young animal which from the time of its birth is able to move about and perform 
voluntary movements of various kinds in a more or less perfect manner, and compare it with the 
helpless new-born human infant which is only capable of exhibiting automatic movements. In 
the former the pyramidal tracts, or motor tracts, which descend from the cerebrum into the cord, 
and which are the paths along which the mandates of the will travel, myelinate at an early period ; 
whilst in the human infant the corresponding fibres do not obtain their medullary sheaths until 
after birth. The study of the dates, therefore, at which the various strands of nerve-fibres 
myelinate not only gives the anatomist a means of locating their position in the white matter of 
the cord, but it also affords the physiologist most important information regarding their functions, 
and also the periods at which these functions are called into play. 
Posterior Column of the Cord and the Posterior Roots of the Spinal 
Nerves.—In the cervical and upper dorsal regions of the cord the posterior column 
is divided by the posterior paramedian septum into the tract of Burdach, which 
lies externally and next the posterior horn of gray matter, and the tract of Goll, 
which lies internally and next the postero-median septum. The tract of Burdach 
is composed of nerve-fibres, which are for the most part larger than those entering 
into the formation of Goll’s tract, and both tracts have a most intimate relation 
to the posterior nerve-roots; indeed, they are both almost entirely composed of 
fibres which enter the cord by these roots and then pursue a_ longitudinal 
course. 
The nerve-fibres which form the posterior nerve-roots, on entering the cord along the 
postero-lateral groove, divide within the tract of Burdach into ascending and descending 
branches. These branches diverge abruptly from each other; and the former take an 
upward course, whilst the latter proceed downwards. The descending fibres are as a rule 
short, and soon turn inwards, to end in the gray matter of the cord. ‘The ascending fibres 
vary greatly in length, and at varying distances from the point where the parent fibres 
enter the cord they turn inwards, to end in the gray matter. A small contribution of 
ascending fibres, however, from each posterior nerve-root, extends upwards to the upper 
end of the cord, to end in the medulla oblongata. 
As each posterior nerve-root enters, its fibres range themselves in the outer part of 
the tract of Burdach close up against the posterior horn of gray matter. ‘The nerve- 
fibres of the nerve-root next above take the same position, and consequently those which 
entered from the nerve immediately below are displaced inwards, and come to lie in the 
tract of Burdach nearer to the mesial plane. This process goes on as each nerve-root 
enters, and the result is that the fibres of the lower nerves are gradually pushed nearer 
and nearer to the postero-median septum in a successive series of lamellar tracts. Of 
course the greater proportion of the fibres, which are thus carried upwards from the 
posterior nerve-roots, sooner or later leave the posterior column and enter the gray 
matter, to end there in relation to some of its cells ; but, as we have said, every posterior 
nerve-root sends a few fibres up the whole length of that portion of the cord which lies 
above, and thus the posterior column gradually increases in bulk as it is traced upwards, 
and in the upper reaches of the cord a tract of Goll becomes evident. This tract of Goll 
is composed of the long ascending fibres of the posterior nerve-roots, which have entered 
the lower segments of the cord. To put the matter differently, the fibres of the sacral 
roots are displaced inwards by the entering lumbar fibres, while the fibres of the lumbar 
roots are in their turn pushed inwards by the entering dorsal fibres, and, lastly, the fibres 
of the cervical: roots displace the dorsal fibres. The difference between the tract of Goll 
and the tract of Burdach simply consists in this, that the former is composed of the fibres 
of posterior nerve-roots which have entered the cord at a lower level than those which: 
enter into the formation of the column of Burdach. The fibres of Goll’s tract, taking 
them as a whole, must therefore necessarily run a very much longer course. 
Our knowledge of the constitution of the posterior columns of the cord is largely 
derived from studying the course of degeneration in monkeys, in which the cord has 
been cut across—either partially or completely. It would appear, from the examination 
of the human cord which has been injured or compressed, that the lamination of the 
