298 ANIMAL PHYSIOLOGY. [LESSON 92. 



stance of the transparent cornea a structure that may (and doubt- 

 less does) prevail to some extent in insects, and justify the peculiar 

 figure of the cornea of the Melolontha vulgaris, given by Straus- 

 Durckheim, and of Libellula by Muller. 



1347. These prismatic, or lenticular bodies, are wider at their 

 anterior than at the posterior extremity ; they possess precisely the 

 appearance that a human crystalline lens, preserved in alcohol, pre- 

 sents they transmit light, but have lost the power of defining ob- 

 jects ; at best, they now possess but semi- transparency. 



1348. It must be understood that this arrangement of lenses 

 (very like a Stanhope lens), in the transparent cornea, is in addition 

 to the double-convex lens, vitreous humor, &c., neither of which 

 were preserved in the beetle in question ; it had once been in spirit, 

 but had become dry long prior to dissection. 



1349. At the margins of the cornea, the lenses have in some 

 instances fallen out, and display the entire depth of it admirably. 

 The horny transparent portion is thin, but the partitions between 

 the transparent portions have great substance, thus leaving deep 

 cells for the reception of the lenticular, prismatic bodies. 



1350. Probably a transverse section of this cornea, undissected, 

 would have presented a figure very similar to that of Straus and 

 Muller, both of whom agree in making this portion of the compound 

 eye of insects of great depth. 



1351. They only differ in one respect, as regards this tissue : 

 Straus says it is plano-convex ; Muller, that it is double-convex. 



. 425. 1352. These preparations, therefore, demon- 



strate that the horny layer which forms the trans- 

 parent cornea is concavo-convex ; instead of being 

 of great depth, as represented, it is a thin layer ; 

 that the interspaces between the facets of this tis- 

 sue descend, to a considerable space, in the pos- 

 terior direction, thereby leaving long and deep 

 cells for the reception of the plano-convex 

 prismatic crystalline bodies ; that these are shut 

 in by a layer filled with round holes, smaller than 

 the plane surface of these prisms ; that the an- 

 terior part of the true crystalline lens passes 

 through these round holes, the large margins of 

 which cut off the light, and form a kind of iris ; 

 and that the posterior portion of the last mei;- 



fiection of the eye of .",., 



Prionus longimanus. tioned lenses, with their deeper curve, touch the 



