Ttciu first descriled. 131 



iheiil, without anj' connection, and the medullary fibres in con- 

 tact with a coloured mucus retained in its situation by its con- 

 ^stence alone. This account is totally at variance with the ge- 

 neral laws of the animal oeconomy: in no instance have we parts, 

 so dissimilar in nature, in actual contact : wherever contact with- 

 out connection exists, each surface is covered by a membrane, 

 from which a fluid is secreted; and wherever parts are united, it 

 is by the medium of cellular membrane, of which serous mem- 

 brane may be considered as a modification. If the retina be 

 merely in contact with the vitreous humour and choroid, we 

 argue from analogy, that a cavity lined by serous membrane ex- 

 ists both on its internal and external surface; but this is not 

 the fact. In the eye a distinction of parts was Tiece!<sary, but 

 to accomplish this a serous membrane was not required ; it is 

 only demanded where great precision in the motion of parts was 

 indispensable, as iti the head, thorax, and abdomen ; a single 

 membrane, with the interposition of cellular su'ostance, answers 

 the purpose here. By this explanation we surmount another dif- 

 ficulty : 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 membrane here, as 

 it is upon the ciliary processes, back of the iris, and pecten, un- 

 der the conjunctiva, round the cornea, and in the edge of 

 the membrana nictit^ns and sheath of the optic nerve in many 

 animals. Dissections are recorded where fluids have been found 

 collected between the choroid and retina, by which the struc- 

 ture of the latter membrane was destroyed : the explanation 

 here given is as sufficient to account for the existence of this 

 fluid, as that which attributes it to the increased secretion of a, 

 serous membrane. 



I take this opportunity of describing the method I adopt for 

 examining and displaying these and other delicate parts, a me- 

 thod which, though simple, will, I expect, prove an important 

 improvement in the means of scrutinizing the structure of animal 

 and vegetable bodies. I procure a hollow s])hpre of glass fro:n 

 two to three inches in diameter, about one-fourth of v/hich is cut 

 off at the part where it is open, and the edges ground^down, so 

 as to fit accurately upon a piece of plate glass, the surface of 

 which is also ground : the object to be examined is attached to 

 a piece of wax fastened upon the plate of glass and immersed in 

 a basin of water, with the cut sphere, which is inverted over it, 

 of course full of water, and the whole withdrawn from the basin. 

 The part may tluls be examined under the most favourable cir- 

 cuumtanccs ; it floats in water, the only method by vwliich deli- 

 cate parts can be unfolded and displayed : the globular form of 



1 2 the 



