444 ME. J. L. CLAEKE ON THE OEET SUBSTANCE OE THE SPINAE COED. 
separated only by blood-yessels which descend from the median fissure ; so that together 
they form but a single mass, along the posterior part of which the gelatinous substance 
is uninterruptedly continued from side to side*. Fig. 2, Plate XX represents a 
transverse section of the grey substance in the dorsal region of the Calf mag^e 
60 diameters. The caput cornu posterioris, on each side, wordd be marked off by an 
imaginary line extending across from the antero-lateral extremity of the gelatinous sub- 
stance to the bottom of the posterior median fissuiu {1 ) ; while the cervicc is^ mcluded 
between this line and another drawn nearly horizontally across from the anterior border 
of the transverse commissure which arches over the pellucid space (H) sun-ounduig the 
central canal. 
The inner or median half of the cervix is occupied hy a remarkable longitudmai 
column, which in this part of the dorsal region is nearly cyhndrical or oval. These 
bodies I described in my first communication, and named them the posterior vesicular 
columns— cohmm vesiculosm posteriores but thek highly interesting structure and 
connexions have induced me to examine them again ivith more attention. 
posterior vesicular column (M, M, fig. 2, Plate XX.) consists of a somewhat dark 
cylinder of fibres interspersed and surrounded by cells and them processes. The fibres 
are in great part derived from the posterior roots of the nerves, and are much finer than 
those of the white columns. They are more or less longitudinal, transverse, and oblique, 
and interlace each other in bundles in the most intricate manner, as may be seen in a 
longitudinal section. 
The cells in the cylinder or opake portion of the column are oval, pyriform, or more 
or less fusiform, and variously stellate (see also Plate XXII. figs. 49 & oO). They (hfler 
considerably in size ; some are very small, but the largest are quite equal to those o t le 
anterior cornu. Their numher, also, varies in different sections of the same thickness, 
and even in the opposite columns of the same section. Sometimes only a few small 
cells, with perhaps a single large one, appear at irregular intervals; and occasionaUy 
they are all absent. Their processes extend transverseli/, oMiquely, and longitudmaUi/ : 
transversely they intersect the cylinder in various ways (M, M, Plate XX. fig. 2), in a 
kind of network formed by their subdivision, and in irregular cmu es, which sometimes 
suddenly change their course to become Imyitiidinal and sometimes escape directly to 
surrounding parts, or after running for some distance round the cylinder. In sertions 
made longitudinally all the cells are seen to be elongated in that diiection (Plate XXII 
fig. 50), and are frequently quite fusiform ; it would therefore appear that each cell is 
prolonged in different planes. They lie imbedded in the plexus of bundles, and bnuich 
into many processes, which run transversely backwards and forwards, oMigiiely and 
longitudmally. Sometimes longitudinal processes extend to a siuprising distance, and 
then become transverse, and often again longitudinal. 
* In the Eodentia-at least in the Guinea-piff, Eat, and Eahhit-the posterior cornna, as in Man, remain 
separate in all regions of the cord. It is cnrious, also, that in these animals the caput cornn is remai-kahh 
large in the dorsal, bnt particularly in the cervical region, and is thronged, as indeed is every otlier part oi 
the grey substance, with small cells. 
