294 7>. Woilastonon [Oct. 



Article XV. 



On Semidecussation of the Optic Nerves. By William Hyde 

 Wo'llaston, MD. VPRS * 



Whether we consider the astonishing subtlety of that 

 medium, which renders visible to us objects existing at the most 

 immeasurable distances from us, or that delicately constituted 

 organ which, by its general structure, collects the rays of light, 

 and by a nice adaptation of its parts concentrates their force on 

 the sentient fibres of the retina, expanded over its inner surface, 

 we can feel no surprise that such great talents should have been 

 devoted to investigate the curious properties of the one, or that 

 the structure of the other should have been examined with so 

 much assiduity. 



The keenness of inquiry manifested by the cultivators of ana- 

 tomy in observing the most minute parts that have escaped the 

 notice of their predecessors, shows that any addition to the 

 common stock of our information on this subject will be gratify- 

 ing to a certain portion of the members of this Society, and 

 probably not uninteresting to the Society at large. 



It is not my object, in the present paper, to examine either the 

 first effect of the cornea in rendering the rays of light conver- 

 gent, or the power of the crystalline lens in finally bringing 

 them to a focus on the retina. It is not my intention to investi- 

 gate whether the adaptation of the eye to different distances is 

 effected by alteration of the form of the lens from its own mus- 

 cular structure, or by alteration of its place, from the agency of 

 other muscles. Nor do I mean to consider either the involuntary 

 motions of the iris dependent on the quantity of light present, 

 or that voluntary contraction of it by which we adapt the aper- 

 ture of the pupil for distinct vision at different, distances, limiting 

 thereby, what in optics is termed the spherical aberration of the 

 lens. 



The subject of my inquiry relates solely to the course by 

 which impressions from images perfectly formed are conveyed 

 to the sensorium, and to that structure and distribution of the 

 optic nerves on which the communication of these impressions 

 depends. 



Without pretending to detect by manual dexterity as an 

 anatomist, the very delicate conformation of the nerves of vision, 

 I have been led, by the casual observation of a few instances of 

 diseased vision, to draw some inferences respecting the texture 

 of that part which has been called the decussation of the optic 

 nerves, upon which I feel myself warranted to speak with some 

 confidence. 



• From the Philosophical Transactions for 1824, Parti. 



