630 
MR. SEWELL ON THE SPINAL MARROW. 
fore the Royal Society, I shall feel myself particularly obliged by 
having so great an honour conferred upon me. 
Upon tracing the sixth ventricle of the brain, which corresponds 
to the fourth in the human subject to its apparent termination, 
the calamus scriptorius, I perceived the appearance of a canal, 
continuing by a direct course into the centre of the spinal marrow. 
To ascertain with accuracy whether such structure existed through¬ 
out its whole length, I made sections of the spinal marrow at dif¬ 
ferent distances from the brain, and found that each divided por¬ 
tion exhibited an orifice with a diameter sufficient to admit a 
large sized pin; from which a small quantity of transparent co¬ 
lourless fluid issued, like that contained in the ventricles of the 
brain. The canal is lined by a membrane resembling the tunica 
arachnoidea, and is situated above the fissure of the medulla, 
being separated by a medullary layer : it is most easily distin¬ 
guished where the large nerves are given off in the bend of the 
neck and sacrum, imperceptibly terminating in the cauda equina. 
Having satisfactorily ascertained its existence through the whole 
length of the spinal marrow, my next object was to discover whe¬ 
ther it was a continued tube from one extremity to the other : 
this was most decidedly proved, by dividing the spinal marrow 
through the middle, and pouring mercury into the orifice where 
the canal was cut across: it passed in a small stream with equal 
facility towards the brain (into which it entered), or in a contrary 
direction to where the spinal marrow terminates. 
By many similar experiments, I have since proved that a free 
communication of the limpid fluid, which the canal contains, is 
kept up between the brain and whole extent of spinal marrow. 
I have consulted the most celebrated authors on comparative ana¬ 
tomy, but do not find any such structure of those parts described; 
and as it is not known to you, l may presume that it has not 
been before taken notice of. 
I have the honour to be, 
Sir, your obedient, faithful servant, 
Wm. Sewell. 
We are really ashamed that we have not earlier put upon 
record, in a veterinary periodical, this important discovery of Mr. 
Sewell, of the central canal of the spinal chord. 
It appeared in the Philosophical Transactions many years be¬ 
fore the exposition of the six columns of that chord by Sir Charles 
Bell. This canal must be the necessary consequence of such a 
construction of the chord. It is singular how often physiologists 
