it >46 3 
V. A Letter on a Canal in the Medulla Spinalis of some Qua - 
. drupeds. In a Letter from Mr. William Sewell, to Everard 
Home, Esq. F. R. S. 
Read December 8, 1808. 
Sir, 
According to your request, I send you an account of the 
facts I have ascertained, respecting a canal I discovered in the 
year 1803, in the medulla spinalis of the horse, bullock, sheep, 
hog, and dog ; and should it appear to you deserving of being 
laid before 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 corre- 
sponds to the fourth in the human subject, to its apparent ter- 
mination, 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 throughout its whole length, I made sections 
of the spinal marrow at different distances from the brain, and 
found that each divided portion exhibited an orifice with a 
diameter sufficient to admit a large sized pin ; from which a 
small quantity of transparent colourless 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 medul- 
lary layer : it is most easily distinguished where the large 
