THE STRUCTURE OF THE SPINAL CHORD. 
611 
caudate vesicles, but in their natural state they certainly bear no resemblance to 
them whatever. Around the extremity of the spongy portion of the posterior cornua, 
these longitudinal fibres are collected into an arched and very dense band, of the 
same form, and about the same thickness as the substantia gelatinosa, with the lower 
border of which it is continuous. This band may be seen even with the naked eye, 
and then appears as a white opake stripe. The longitudinal fibres are also found, 
but in smaller numbers, through the rest of the grey substance. 
Changes in the Form of the Grey Substance, 
On examining the chord upwards towards the lumbar enlargement, certain modi- 
fications are found to take place in the form and disposition of the grey masses. The 
two halves of the posterior mass, with its two sets of antero-posterior bundles of 
tubular nerve-fibres, begin to separate from each other at the middle line (see figs. 2 
and 3 ). Here the substantia gelatinosa is first interrupted, and the posterior border 
of the grey substance between its divided central extremities, or rather, the posterior 
fibres of the transverse commissure, begin to arch forwards towards the spinal canal, 
and to mark out the rudiments of the posterior cornua. At the same time, on each 
side, and a little behind, the spinal canal, a small but gradually increasing mass of 
caudate vesicles makes its appearance. Tliese are the commencement of two longi- 
tudinal columns of vesicular substance, which extend through the whole length of 
the spinal chord, and which I name the posterior vesicular columns. They are 
traversed and surrounded by fibres from the posterior roots of the nerves, and also 
by arched fibres of the posterior transverse commissure, some of which, as already 
stated, are continuous with the anterior roots of the nerves. The changes just de- 
scribed continue to increase from below upwards, and reach their greatest extent in 
the middle of the lumbar enlargement, where the posterior cornua are broad and 
long, and widely separated from each other by the posterior white columns of the 
chord ; while the space between the posterior border of the transverse commissure 
and the spinal canal, and which is almost entirely occupied by the former structure, 
is much reduced in breadth, for it measures only the ij^th of an inch, whereas at 
the lower extremity of the chord its diameter reaches the 3-oth of an inch (see Plates 
XX. and XXI. figs. 1 and 6). At this central constricted portion, therefore, of the 
transverse commissure, its fibres, which before were but loosely arranged, are now com- 
pressed into a dense band, but still diverge on each side into the anterior and posterior 
cornua. In consequence of these changes, also, the two posterior vesicular columns, 
which have increased considerably in size, and were formerly situated behind, and 
at the sides of, the spinal canal, are now pushed up, as it were, and included in the 
presenting a very small diameter and a bright golden colour ; but add, that they are not easily examined with 
accuracy. Repeated and careful examination, however, has convinced me that the above description is correct; 
their golden colour, when seen by transmitted light, is due only to the greinular matrix in which they lie ; when 
viewed by direct light, they have an opake white appearance. 
