1 14 Memoir upon the compound 



111 order to observe these white points, or the extremities of 

 the nervous filaments which compose the particular retinae of 

 each facet, the cornea must be carefully renjoved, operating its 

 section from the outside to the inside, and taking care not to 

 remove the coating.';; for, however little we disturb the filaments, 

 they are so contractile, that they close in upon the optic nerve, 

 and no longer appear on the coating of the cornea. If, instead 

 of dissecting the eye from the outside to the inside, we observe 

 it by successively removing the internal parts, we can never dis- 

 cover this arrangement, even in the genera where it is most de- 

 cided; such as the grylliis, the mantis, the libelliila, and the 

 tabanus. This depends on the contractibiiity of the optic 

 ner\'es, which is sometimes so great as to draw the filaments even 

 bevond the choroid. 



Immediately under the optic filaments and the coating of the 

 cornea we observe the coating of the choroid. This coating or 

 varnish is a viscous substance, not liquid, soft, very clammy, and 

 not very soluble in water. It is also strongly adherent to the 

 membrane vs'hich it covers when the latter exists. Svvammer- 

 dam remarks with great truth, that this opaque varnish stains 

 the fingers like the common pigmentum nigrum. But this ac- 

 curate observer seems to have confounded the coating of the 

 cornea w-ith the varnish of the choroid, when he says thut the 

 latter is variously coloured according to the species. On the 

 contrarvj the black colour and the opacity of the varnish of the 

 choroid' are established beyond contradiction. I'his colour even 

 seems to be internally connected with the texture of the choroid, 

 for it is impossible to discharge it by repeated macerations. 

 Contrary to the opinion of Hooke*, Swammerdam thinks that 

 there does not exist in the eyes of insects any humour properly 

 so called ; and in fact, neither the tunic of the cornea nor tlie. 

 varnish of the choroid deserves the name of humour, particularly 

 if W'C compare them to what is understood by the term humour 

 in the eves of vertebral animals. Finally, the same anatomist 

 regards the blackish varnish of the choroid, as the extremity of 

 certain fibres placed immediately under the cornea, fibres which 

 have been torn on removing this membrane. This explanation 

 appears very imjirobable, since, when the tunic of the cornea is of 

 a colour different from the varnish, there never exist any blackish 

 fibres corresponding with this same cornea. Besides, how could 



■* liooke and Boyle were tlie first who inriiiitainerl that air was necessary 

 to coinbusUoii and lespiiation, and that those operations consume but a 

 certain portion of it. ilooke even conjectured that air was fixed in nitre, 

 and that combustion was a chemical |)roccss, i. e. the solution ot' the burn- 

 jns; bodv in a fluid, or.its union with this substance. 'I'lic chemists of the 

 jireseut day liold no other language. Mkro^ra^lua, pp. 45. 104, 105. 



the 



