ME. J. L. CLARKE ON THE GREY SUBSTANCE OF THE SPINAL CORD. 451 
that in the upper part of the dorsal region (compare with fig. 6, Plate XXL). This 
traet is traversed by several roots of the spinal-accessory nerve, in their course forwards to 
the anterior cornu, and contributes, with the edge of the posterior cornu, to form a beautiful 
network (N, N, N) in the lateral column, through which the nerve enters. Its cells are 
triangular, ot'al, and fusiform in ditferent directions : some of them are elongated in the 
direction of the ti-ansverse commissure and towards the front of the posterior vesicular 
column; others extend outwards with radiating fibres through fissures of the lateral 
column. There is reason, therefore, to believe that this tract forms a part of the tractus 
intermedio-lateralis. In the Sheep and Ox, and probably in all Mammalia, a peculiar 
group of cells, which is traversed by the roots of the spinal-accessory nerve, is found in 
the same situation ; and this group, in ascending the medulla oblongata, retires inwards 
to the space behind the canal, and there contributes to form the nucleus which gives 
origin to the highest roots of the ner\ e. It has also been seen that the cells of the tractus 
intermedio-lateralis are elongated with their processes in a longitudinal direction, and 
reached by both the posterior and anterior roots of the spinal nerves, and perhaps by 
the spinal-accessory ; that the latter nerve extends forwards to the cells of the anterioi' 
cornu, which also send some of their processes longitudinally, and are reached by the 
postenor roots. Moreover, I have in another memoir showoi that, while one portion of the 
upper roots of the spinal-accessory nerve and one portion of the vagus roots proceed 
inwards to their respective nuclei behind the canal, other portions of both bend forwards 
to the vesicular network into which the anterior cornu has become resolved'^. Again, I have 
shown, in the same memoir, that some of the roots of the trifacial nerve descend longi- 
tudinally through the caput cornu, betwnen the transverse roots of the vagus-, in which 
course they are probably brought into connexion with the respiratory centres, and perhaps 
also, like the vagus, wuth the anterior grey substance of the medulla. These extensive 
and intimate connexions seem to afford an explanation of the mechanism by which 
impressions made on the vagus and on the incident fibres of the trifacial and spinal nerves, 
may call into action the whole class of respiratory muscles ; and if the tract which I have 
just described in the upper part of the ceinical region be continuous, as it probably is, 
with the tractus intermedio-lateralis, which is reached by the dorsal nerves supplying the 
i)itercostal and other respiratory muscles of the trunk, the explanation in question will 
be still more completef . 
I shall now briefly trace the posterior vesicular columns of the human cord from the 
dorsal region downwards. At the upper part of the lumbar region, as in Mammalia, they 
are larger than in any other portion of the cord (see fig. 1 3, M, Plate XXI. ). They have 
the same form and dark appearance as in the dorsal region, but contain more large cells. 
As they descend, however, the dark masses diminish, and are frequently broken into 
* See the author’s “ Aledulla Oblongata,” Phil. Trans. Part I. 18.38, pp. 252, 253, figs. 19 and 3G. 
t Both in this communication and that on tlie Medulla Oblongata, I have abstained from drawing any 
physiological conclusions, except on this one occasion, where so many different facts required to be brought 
together while fresh in che memory of the reader. 
