Mr. Sewell's Letter , &c. 
H7 
nerves are given off in the bend of the neck and sacrum, im- 
perceptibly terminating in the cauda equina. Having satisfac- 
torily ascertained its existence through the whole length of 
the spinal marrow, my next object was to discover whether 
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 termi- 
nates. 
By many similar experiments, I have since proved that a 
free communication of the limpid fluid, which the canal con- 
tains, is kept up between the brain and whole extent of spinal 
marrow. I have consulted the most celebrated authors on 
comparative anatomy, but do not find any such structure of 
those parts described, and as it is not known to you, I may 
presume that it has not been before taken notice of. 
I have the honour to be. 
Sir, your obedient faithful servant, 
WM. SEWELL. 
Veterinary College, 
Nov. 26, 1808, 
