486 Dr Alison on Single and Correct Vision, by means of 



known, not only that the endowments of different nervous fila- 

 ments, of precisely the same structure, and contained in the same 

 mass, or bound in the same sheath, are often perfectly different ; 

 but even that different portions of the same nervous filaments, 

 especially where they connect themselves with the central masses 

 of nervous matter, may have perfectly different endowments ; as, 

 e. g. in the case of the fibres ascending from the Corpora pyra- 

 midalia, through the crura cerebri and corpora striata, to the 

 bottom of the convolutions, the whole of which are continuous, 

 but only part of which possess the power of exciting muscular 

 contraction when irritated. 



Now, if we inquire in what part of the contents of the cra- 

 nium the sensations of the eye are found by experiment to reside, 

 the experiments of Mayo and of Flourens, particularly those of 

 the latter author, which were repeated before, and reported on 

 by Cuvier, are generally regarded as affording satisfactory evi- 

 dence that they reside in the Corpora quadrigemina. These 

 bodies, and the optic nerves, appeared distinctly to be the only 

 parts of the nervous system, the irritation of which uniformly ex- 

 cited the contraction of the iris, no doubt by producing a sensa- 

 tion of light ; and the destruction of which uniformly stopped the 

 play of the iris, and extinguished vision.* 



Our business is, therefore, to learn in what manner those fibres 

 of the Tractus optici, which can be distinctly traced into the 

 Corpora quadrigemina, are there implanted ; and when we trace 

 the course of these fibres in the brains of the mammalia (harden- 

 ed by alcohol), whether they descend on the Corpora quadrige- 

 mina from the Thalami, or pass more directly backwards below 

 the Corpora geniculata, it seems to me quite obvious that they 

 first turn inwards, and then enter the Corpora quadrigemina 

 from above downwards, and are so expanded over the superior of 



* See Recherches Experimentales, &c. p. ]50, et seq. 



