132 Animal Morphology. 



IV. 



THE SENSE OF SIGHT. 



The Essential Tripartite Membrane. 



Mucous Membrane. Membrana pigmenti, alias Choroid 



membrane. 



Serous Membrane. Jacob's membrane. 



Muscular Membrane. The ciliary muscle and ligament, the 

 latter being probably modified muscle. 



Supplementary Tripartite Membrane. 

 Mucous Membrane highly modified. Cornea. 

 The Muscular Membrane. The oblique and recti muscles, 



with their membranous expansion, the sclerotic coat; 



the orbicularis palpebrarum and the levator palpebrse ; 



and the corrugator supercilii displaced. 

 Serous Membrane. Crystalline lens, and aqueous and vitreous 



humours. 



The highly modified and differentiated condition of the 

 paraitic senses, and proneness to displacement inter se y 

 renders it extremely difficult to trace their relations and real 

 locality, since each in its function is so much aided by one 

 in close relation to it. And as in the brain some appear to 

 combine, as it were, by commissure, and to blend the pro- 

 ducts of their various stimuli by a peculiar interlacing of 

 fibres reaching to certain cineritious cells or gre,y substances, 

 so, in their external apparatuses the tripartite membranes of 

 these senses appear to interchange one with another. 



As the sense of smell supplies to the eyes and ears a 

 certain amount of mucous membrane, as in the lachrymal 

 apparatus, and to the tympanum and Eustachian tube in the 

 ear, so the sense of taste supplies, in its serous membrane, the 

 arytenoid cartilage and epiglottis, which are regulated by 



