150 ORGAN OF SIGHT. 



CHAPTER XII. 

 OKGANS OF SPECIAL SENSE. 



Organ of Sight. The epithelium, cellular elements, and 

 the finer nerves of the cornea have been already treated of 

 (Chapters II., III., and V.). We have only to remark that, 

 in order to observe the relation of the cornea to the conjunc- 

 tiva, sclerotica, and ligamentum pectinatum, it is necessary to 

 harden the bulb entire, and to make sections which shall in- 

 clude all these structures. The best and simplest method 

 consists in placing the fresh bulb of a mature foetus, a rabbit, 

 a pig, or a calf, in one-tenth per cent, solution of chromic acid, 

 for eight or ten daj^s ; having previously made two or three 

 punctures in it with a lancet-shaped needle. After two or 

 three da} 7 s of immersion, the bulb may be cut in two with a 

 razor, the crystalline lens and vitreous body removed with 

 forceps, and the anterior half of the bulb (containing the con- 

 junctiva, the cornea, the iris, and processus ciliares, the an- 

 terior segment of the sclerotic and choroid as well as the ora 

 serrata retinae, and zonula Zinii) put back in the solution. 

 The necessary consistence having been obtained, a portion is 

 cut off in such a way as to render it possible to make trans- 

 verse sections through the above-mentioned structures. The 

 sections may be treated in the ordinary way ; and, if thin 

 enough, they will also be useful for the purpose of studying 

 the tissue of the sclerotic, choroid, ciliary processes, and iris, 

 as well as of the musculus tensor choroidese. 



The cellular elements of the sclerotic may be further de- 

 monstrated in surface preparations as follows : The bulbus oculi 

 of a frog having been extirpated, is carefully freed from ad- 

 herent connective tissue on an object-glass ; the surface of the 

 sclerotic is then thoroughly touched with lunar caustic : after 

 a quarter of an hour small portions are cut off: these must be 

 pencilled on their inner surface with a camel-hair brush, so as 

 to remove any adhering pigment ; the preparations being 

 finally mounted in glycerin. Successful preparations exhibit 

 branched clear spaces canaliculi on a brownish-ground, such 

 as have been previously described. 



Other portions of fresh sclerotic may, after pencilling, be 

 treated with a half per cent, solution of chloride of gold, and 

 employed both for vertical and horizontal sections. In the 

 former, the violet-colored cellular elements appear as spindle- 



