CHAP. XL] PERIPHERAL NERVOUS END-ORGANS. 4G9 



pigment occurs in the form of elongated, sometimes spindle-shaped, 

 rods in the epithelium cells; these rods are protruded into the 

 processes of the cell protoplasm. 



Method of More than 500 hens' eyes are bisected and the 



separating posterior halves are placed whilst yet fresh in alcohol ; 

 Fuscin Mays' they are then boiled in alcohol and afterwards extracted 

 with boiling ether and water. They are then subjected 

 to energetic trypsin-digestion for 24 hours. The pigment is then left 

 in little masses which are collected on gauze and triturated with alkali 

 with the object of separating nucleins. It is mechanically separated 

 from adherin neurokeratin. 



Pro erties ^" c ^ em ' ca ^ reagent dissolves fuscin, except con- 



centrated acids and alkalies, and these only do so very 

 gradually, or by the aid of heat. On long boiling in concentrated 

 sulphuric acid fuscin dissolves, colouring the acid of a dark brown 

 colour. By long digestion in caustic alkalies and their carbonates, 

 fuscin dissolves. 



In the presence of oxygen, fuscin is slowly bleached, apparently in 

 consequence of an oxidation process ; the sensibility of the pigment 

 obtained from different animals appears to differ. 



Fuscin is a nitrogenous body and, on ignition, leaves a small 

 quantity of ash containing iron. 



Action of Light upon the Visual Purple of the Living Eye. 

 Regeneration of Visual Purple. 



Though when the eye is exposed to diffuse daylight the visual 

 purple is not destroyed, by exposing frogs for considerable periods to 

 direct sunlight the retinae are found to have been bleached. Byallo wing 

 such frogs to remain in comparative darkness the colour is however 

 soon restored. Amongst the earliest of Kiihne's experiments were 

 those which threw light upon the structures which retard the bleaching 

 of visual purple or are concerned in its restoration. 



If an equatorial section be made through a recently extirpated 

 eye, and a flap of retina be lifted up from the underlying choroid to 

 which the retinal epithelium cells are adhering, and if the whole be 

 exposed to light, it will be found that the purple colour of the flap 

 will be destroyed, whilst the colour of the rest of the retina will 

 persist. If, however, the bleached portion of flap be carefully 

 replaced, so that it is again in contact with the retinal epithelium 

 cells, complete restoration of the visual purple occurs. The restora- 

 tion is a function of the living cells, and it appears to be independent 

 of the fuscin which they contain. As it is absolutely dependent on 

 the life of the structures which overlie the rods, it is natural that, it 

 should persist for a longer time after somatic death in the frog than in 

 the rabbit. 



