116 Memoir upon the compound 



which result fasten on the edges of the eye : the latter form the 

 large circular trachea, unite afterwards to the upper part, con- 

 tinuing after this junction in one single trachea, which, by join- 

 ing to another, afterwards terminates at the base of the brain. 



The large circular trachea furnishes an infinite number of 

 very minute tracheae, which soon becoming bifurcated, form verv 

 numerous isosceles triangles which rest on the circumference of 

 the optic cone. These triangles formed by the tracheae are di- 

 vided as by a perpendicular, by the nervous filaments resulting 

 from the expansion of the optic nerve. These filaments after- 

 wards pass through the choroid and its opaque varnish, as well 

 as the tunic of the cornea, and terminate below the facets of 

 the last-mentioned membrane. This arrangement of the trachefp 

 and of the nervous filaments forms a handsome network, which 

 is rendered verv sensible by carrying the nerve inwards, and on 

 the side of the brain. All these tracheae afterwards continue, 

 and terminate on the choroid. The genera which have no 

 choroid also want the circular trachea. 



In the genera which have vesicular tracheae, like the lamelli- 

 corn coleopierce, most of the lep'idoplerce and d'ipterce, as well 

 as certain orthoptercB, like the gryllus and the tntxalis, we 

 observe considerably under the optic cone another circular tra- 

 chea, but much smaller than that which surrounds the edge of 

 the cornea. The latter turns around the optic nen-e, and is 

 surrounded itself by numerous air pouches, the use of which 

 seems to be, to sustain the optic nerve and to keep it in its position. 



The small circular trachea is wanting in all the genera which 

 do not present pneumatic pouches or vesicular tracheae: as it 

 appears, however, essential for keeping the nerve in its position, 

 it is replaced by the fibres of the adductor muscle of the mandi- 

 bles, which on separating wholly surround the optic nerve, 

 and prevent its being put out of place. We cannot say that 

 the muscle in its contractions can act on the nerve by com- 

 pressing it; for observation proves, that by placing the muscle in 

 all the contractions of which it is capalile, the nerve remains al- 

 ways in its natural position, since the contraction of the muscle 

 is effected longitudinally only, and from front to rear; so that, 

 whatever contraction it undergoes, it can never touch the optic 

 nerve. 



The optic nerve formed by the prolongation of the brain is, 

 of all the nerves of the head, the largest and broadest, particu- 

 larly if we measure it at the place where it spreads. It issues 

 almost always from the lateral and upper surfaces of the brain ; 

 but its position with respect to the other nerves furnished by 

 the brain is very variable. According to the species and position 

 •f the various parts situated in the head, it is either the third, 



the 



