Dr. Jacob’s account of 
30 6 
described as being extended between the vitreous humor and 
choroid, from the optic nerve to the ciliary processes, being 
merely laid between them, without any connection, and the 
medullary fibres in contact with a coloured mucus retained in 
its situation by its consistence alone. This account is totally 
at variance with the general laws of the animal economy : 
in no instance have we parts, so dissimilar in nature, in actual 
contact: wherever contact without connection exists, each sur- 
face is covered by a membrane, from which a fluid is secre- 
ted ; and wherever parts are united, it is by the medium of 
cellular membrane, of which serous membrane may be con- 
sidered as a modification. If the retina be merely in con- 
tact with the vitreous humor and choroid, we argue from 
analogy, that a cavity lined by serous membrane exists both 
on its internal and external surface ; but this is not the fact. 
In the eye a distinction of parts was necessary, but to accom- 
plish this a serous membrane was not required ; it is only 
demanded where great precision in the motion of parts was 
indispensable, as in the head, thorax, and abdomen ; a single 
membrane, with the interposition of cellular substance, answers 
the purpose here. By this explanation we surmount another 
difficulty, the unphilosophical idea of the colouring matter 
being laid on the choroid, and retained in its situation by its 
viscidity, is discarded ; as it follows, if this account be correct, 
that it is secreted into the interstices of fine cellular mem- 
brane here, as it is upon the ciliary processes, back of the iris, 
and pecten, under the conjunctiva, round the cornea, and in 
the edge of the membrana nictitans and sheath of the optic 
nerve in many animals. Dissections are recorded where 
fluids have been found collected between the choroid and 
