NEW PROPERTY OF THE RETINA. 329 



causes, it is likely to extend itself; if from the second, it may remain as it is. 

 Having observed it only for a year without noticing any enlargement, it is 

 probable that it was produced by the strong action of light. 



Owing to the compound structure of the retina, consisting of different layers, 

 and these layers composed of bodies of different shapes, it is very difficult to dis- 

 cover the part which each of them performs in the act of vision ; but considering 

 each element of the retina as a rod, the end of which next the vitreous humour 

 is an expansion of the optic nerve, we know that distinct vision of external 

 objects arises from the law of visible direction, by which every ray of light, at 

 whatever angle it may fall, gives us vision of the point from which it proceeds, 

 in a direction perpendicular to the part of the membrane on which it is incident. 

 When this outer layer of the retina is insensible to the light of external objects, 

 its luminosity, or the light which it exhibits, may be received from the surround- 

 ing parts of the expanded nerve by irradiation, or from the parts of the elemental 

 rods behind it, if they were not paralysed, or if they are, by the action of the un- 

 paralysed rods around them. 



Although in hemiopsy, and in the case of local amaurosis which I have 

 described, the paralysed parts are still luminous, yet there are cases in which these 

 parts are absolutely black, and into which no light is introduced by irradiation. 

 An example of this fact presented itself to me in the 

 morning of the 16th October 1837, and is represented in 

 Fig. 3, where two black curved lines proceeded from the 

 foramen centrale of the retina of the right eye. These 

 lines were so black that, in the memorandum which I 

 made at the time, I state that they were blacker than 

 the black ink lines upon the paper. The lines continued 

 only about ten minutes, and were probably produced by 

 the pressure of blood-vessels, as I had, the day before, ^mTz 



been subject to much giddiness. In this case, the elemen- 

 tary rods of the retina beneath these lines must have been paralysed throughout 

 their length ; and, therefore, it is probable that in the cases of hemiopsy and 

 local amaurosis, the paralysis affects only the end of the rods in contact with 

 the vitreous humour, and formed by the expansion of the optic nerve. 



In concluding this notice, T would suggest to philosophers and medical prac- 

 titioners the importance of studying the manner in which sight and hearing 

 are, in their own case, gradually impaired, for it is in the decay or decomposition 

 of organic structures, as well as in their origin and growth, that valuable 

 results may be presented to the physiologist ; and facts of this kind have a 

 peculiar value when the patient is himself a practised observer. 



