4:)8 THE SENSORIAL FUNCTIONS. 



enclose being occupied each by a transparent 

 cylinder (v), consisting of an outer membrane, 

 filled with a viscid substance analogous to the 

 vitreous humour. Their general form and situa- 

 tion, as they lie embedded in the pigment, may be 

 seen from the magnified sections ; each cylinder 

 commencing by a rounded convex base, imme- 

 diately behind its respective pupil, and slightly 

 tapering to its extremities, where it is met by a 

 filament (n) of the optic nerve ; and all these fila- 

 ments, after passing for a certain distance through 

 a thick mass of pigment, are united to the large 

 central nervous bulb (g, Fig. 427), which is termed 

 the optic ganglion.^ 



It thus appears that each of the constituent eyes, 

 which compose this vast aggregate, consists of a 



* Numberless modifications of the forms of each of these con- 

 stituent parts occur in different species of insects. Very frequently 

 the vitreous humour (v), instead of forming an elongated cylinder, has 

 the shape of a short cone, terminating in a fine point, as shown in 

 Fig. 426. Straus Durckheim appears to have mistaken this part 

 for an enlarged termination of the optic nerve ; believing it to be 

 opaque, and to form a retina applied to the back of the corneule, 

 which latter part he considered as properly the crystalline lens. In 

 his elaborate work on the anatomy of the Melolontha, he describes 

 the filaments (f) of the optic nerve, in their progress inwards, as 

 passing through a second membrane (k, Fig. 428), which he deno- 

 minates the common choroid, and afterwards uniting to form an 

 expanded layer, or more general retina (r), whence proceed a small 

 number of short but thick nervous columns (s), still converging 

 towards the large central ganglion (g)^ in which they terminate. 

 The use he ascribes to this second choroid is to intercept the light, 

 which, in so diminutive an organ, might otherwise penetrate to the 

 general retina and produce confusion, or injurious irritation. The 

 colour of the pigment is not always black, but often has a bluish 

 tint : in the common fly, it is of a bright scarlet hue, resembling 

 blood. In nocturnal insects, the transverse layer of pigment between 

 the corneule and the vitreous humour is absent. 



