280 Memoir vpon the cowpound 



hurtful to vision. Thus it is particularly by the privation of thi* 

 membrane, that we may judge of its influence on the sense of* 

 sight ; and observation jiroves that it is wanting in all the lucifugcB. 



According to this way of considering the varnish of the cho- 

 roid and the choroid itself, we see that the opacity of the latter, 

 alwavs very great, may be no obstacle to vision. It is even pos- 

 sible that this membrane should serve to render it more com- 

 plete, bv preventing the sensibility of the optic nerves from be- 

 ing affected by the almost immediate action of the light on their^ 

 texture. 



As to the air tracheae, their chief use seems to be to sustain 

 the spreading of the optic nerve, and to circumscribe it within 

 limits essential to the mechanism of vision. Besides, it is 

 more than probable that the choroid is formed by a heap of 

 tracheae furnished by tlie large trachea which surrounds it. Thes«. 

 trachece, which are distributed over the cellular texture of the 

 choroid, adhere bv its upper surface to the varnish, and by its 

 inferior to the spreading of the optic nerve. This organization 

 seems even to be proved by tbe examination of these various 

 parts. The large circular trachea, placed almost on the same 

 plane with the choroid, furnishes in its texture a very great num- 

 ber of small tracheae which lose themselves in it, since we can 

 find no trace of them after this membrane. Finally, by a pro- 

 longed maceration, we may assure ourselves that the choroid is 

 formed by a close texture, of which, perhaps, the great number 

 ©f tracheae which lose themselves in it, is the cause. We might 

 also remark that, in insects, the trachea;, in being distributed 

 over the cellular texture, form by tlieir union with this texture 

 various membranes, and even several of a considerable extent. 

 The cellular tunic alone of the intestinal tube is a proof of this: 

 it is singular enough that the choroid, formed in the red-blooded 

 airimals by a triple vascular texture, is, of all the parts of insects, 

 that which receives most trachos. In short, the choroid is not 

 the only membrane of insects in which we might find a similar 

 resemblance. As the air is tlie only fluid which has any circula- 

 tion in them, all the parts which ought to liave either a great 

 sensibility, or a great contractibllity, generally present a great 

 number of tracheae. Thus we may always estimate a priori, 

 either the force of a muscle, or the sensibility of an organ, by the 

 tracheas which wc see them receive. 



The situation cf the optic nerve, with respect to the eye, seems, 

 of no consequence to insects. In fact, the numerous nervous 

 filaments corresponding to the facets of the cornea, render it in- 

 different that the rays of light should fall with more or less ex- 

 actitude on the insertion of the optic nerve, since the latter can 



only 



