27G Answer lo Ohjecllons against Mr. Horn's Theory of Fision. 



" It is evident," says Dr. Reid, " that the pictures upon the 

 retina are by the laws of Nature a mean of vision ; but in what 

 way they accomplish their end we are totally ignorant." 



From the different hypotheses respecting the seat of vision, 

 and contradictory opinions as to the sensibility of the retina, the 

 immediate inference that every unprejudiced inquirer must draw, 

 is, that their respective authors were alike uncertain as to the* 

 structure and use of this membrane : — each found himself per- 

 fectly at liberty to accommodate its character to his own hypo- 

 thesis. 



But whatever the difficulties may be respecting its texture, 

 the nervous nature and origin of the retina are supposed demon- 

 strable from the constitution of the optic nerve. It is allowed 

 by anatomists, that the nerve possesses two tunics that envelop 

 its medullary substance ; — the exterior, derived from the dura 

 mater, forming by its expansion the sclerotic coat of the eye ; 

 and the interior, which is a continuation of the pia mater, ex- 

 pands itself on entering the eye, and forms the choroides. The 

 retina or innermost coat of the eye is therefore sujjposed to be 

 a propagation of the nervous substance ; the entire trunk of the 

 optic nerve being thus naturally expanded into the principal 

 coats that compose the globe of the eye. 



Now, however natural this arrangement of parts may appear, 

 yet I conceive the fact, as far as it relates to the retina, to be 

 very questionable. 



In the first place, the controversy so ably carried on, without 

 clearly deciding the question, whether the retina or the choroides 

 be the principal organ of vision, leaves the character of the for- 

 mer in suspense. To say nothing of those inadequate hypotheses 

 which Pecquet was forced to invent, in order to account for its 

 insensibility upon the base of the nerve, some of its advocates 

 strenuously contend, that the retina is every where insensible, 

 except at the axis of the eye. Others, with equal confidence, 

 afiirm that it is totally insensible ^t the axis, and confine its sen - 

 sibility to the anterior portion. On tire contrary, Marriotte and 

 others, who plead for the choroides, assert that the retina is 

 insevsible tiirougkout its ivlwle structure. Le Cat, who held 

 that the pia mater, and not the nerves themselves, is the proper 

 instrument of sensation, supposes that the retina answers a pur- 

 pose similar to Uie scarfskin covering the papiUcB pijramidaleSy 

 which are the innnediate organ of feeling. The retina, he says, 

 receives the impressionsof the rays, moderates and prepares them 

 for the proper organ, but is itself insensille to any impression 

 from the rays of light. 



In the second place, it appears highly unreasonable to sup- 

 pose so large a mass, as the medullary substance of the optic 



nerve. 



