74 Miscellaneous, 



diiced to explain the phenomenon of vision. As the author points 

 out, the case of albinos shows that luminous perceptions do not 

 cease when the pigment is absent ; and we do not know any case in 

 which the pigment is absolutely necessary for the perception of light. 

 The pigment really fulhls two functions. In the first place it 

 absorbs the superfluous light and prevents it from being reflected 

 upon other parts of the retina ; and it arrests all the luminous rays 

 which may penetrate into the eye by any other road but the pupil, 

 whether through the cornea or the sclerotic. This second function 

 is of particularly great importance in the Mollusca, In many of 

 these (for example, in the Heteropoda) the parts surrounding the 

 eye are jjerfectly transparent ; and even in the snails the ommato- 

 phore is sufficiently transparent to aUow us sometimes to see the 

 outer layer of the retina without any difficulty. The luminous rays 

 may, therefore, strike this outer layer of the retina in all directions ; 

 and from this M. Heusen justly concludes that it cannot be sensitive 

 to light. Sensibility to luminous rays consequently appears to be 

 peculiar to the inner layer, accessible only to the rays which 

 have passed through the crystalhne. This layer alone is com- 

 parable to the stratum of bacilli in the Vertebrata. 



In comparing the eyes of the Mollusca with those of other 

 animals, M. Hensen directs attention to the difficulty resulting from 

 the variable meanings of terms. The words eye, retina, iris have 

 acquired a perfectly definite physiological sense ; but this is not the 

 case wdth the words sclerotic, cornea, and choroid, because they are 

 used to designate organs with various and still LU-defined functions. 

 Thus, for example, the sclerotic serves at once as the protective en- 

 velope of the eye, as the support of the cornea, and as the basis for 

 the attachment of muscles, without its being possible to say that 

 any one of these functions is more essential than the rest. In the 

 Vertebrata the sclerotic and the neurilemma of the optic nerve are 

 justly regarded as prolongations of the ditra mater. This morpho- 

 logical character ought, apparently, to be the best guide in the in- 

 vestigation of the homologies of the sclerotic ; but when we come 

 to animals in which the eyes are not formed as if by a budding of 

 the brain, and in which we can find no dura mater, it becomes very 

 difficult any longer to speak of a sclerotic. 



With regard to the structure of the retina there is, between the 

 Vertebrata and the Invertebrata, a chasm which seems to defy all 

 homologies. Thus in the former the bacilli form the outer layer 

 of the retina ; in the latter they form its inner layer. And yet it 

 is remarkable that, notwithstanding this fundamental difi'erence in 

 the typical organization of the essential part of the organ of sight, 

 the uniformity of organization persists in the accessory organs. 

 Thus, as M. Hensen remarks, in the Cephalopoda, the crystalline 

 continues to be an epithelial production, resulting from an invagi- 

 nation of the skin, as in the Vertebrata. In these higher Mollusca 

 there also exist an iris, a cornea, and eyelids — organs which, it is 

 true, disappear one after the other in this class of animals. The 

 sclerotic appears as if divided into fragments. One portion forms the 



