BIOLOGY. 



[BOOK vi 



with single cornea, of many arachnida. \Ve still therefore find 

 here an essential uniformity under an apparent variety. 



But we can generalise still more. In effect, there is a great 

 analogy of conformation in the terminations of the sensitive 

 nervous fibres. For the optical, auditory, olfactory fibres the 

 morphological resemblance is striking. Each of these fibres 

 ends, at the periphery, in a cell connected with a special organ, 

 or bacillum, whose function probably is to select among the 



FIG. 82. 



Transversal and horizontal section, magnified, of the globe of the eye ; a, sclerotic ; o, cor- 

 nea ; c, lamella of the conjunctiva passing over the cornea ; d, circular vein of the iris ; 

 e, choroid with its pigmentary layer ; /, ciliary muscle ; g, ciliary process ; h, iris in the 

 middle of the pupil ; I, optical nerve ; k, anterior edge of the retina (ora serrata retinae) ; 

 I, crystalline lens surrounded by its capsule ; m, membrane of Descemet lining the in- 

 terior chamber of the eye ; n, internal layer of the retina; o, membrane of the vitreous 

 body; p, canal of Petit. 



molecular vibrations of the ambient world those which are 

 adapted to the special sensibility of the fibre which it terminates. 

 The organs of the senses with complicated structure, such as 

 the eye and the ear, are only accessory apparatus, which render 

 more easy the function of the bacillum, by concentrating on it 

 such or such a kind of excitation. In effect in proportion as we 



