446 PHILOSOPHICAL TRANSACTIONS. [aNNO I67O. 



rays which come from one point in the axis, and pass near the centre of the 

 crystalline, to refract more than if there had been but one crystalline. And by 

 these means the greatest part of the rays, which fall upon the extremity of the 

 great crystalline, intersect the axis; which causes their sight to be less confused, 

 although it be never so distinct as that of men and birds, which have but one 

 crystalline. Fishes have also a double crystalline, for otherwise their sight 

 would be more confused than that of other animals who live in the air, for their 

 crystalline being spherical, the rays cut the axis more unequally than if it were 

 lenticular, and its convexities were of a greater sphere; and it ought necessarily 

 to be spherical, because the refraction of the rays, which pass from the water 

 into the crystalline, is very small, and would make its focus too far distant if 

 the crystalline were lenticular. 



The difficulty of your second objection proceeds also from an ambiguity of 

 words, and consists in knowing what we are to call that which has a great conti- 

 nuity and communication with the brain. My hypothesis is, that the nerves 

 are all coated with the pia mater, (which envelopes all the spinal marrow) and 

 have with it the same continuity of fibres; so that if these nerves be never so 

 little moved, the impression is carried to the brain by the continuity of these 

 fibres; and, whether it be that the texture is different in nerves of different 

 senses ; or that they contain some spirituous liquors which determine their sensa- 

 tions, by some differences they have among themselves ; it is certain, that the 

 nerves of sight, in what manner soever they are moved, represent colours and 

 light, those of hearing sounds, and those of the touch, pains, &c. 



Now the choroides is an expansion and dilatation of the pia mater, which 

 envelopes internally the optic nerve, and which comes from the tuberosity of 

 the spinal marrow by a continuity of fibres; whence it follows, that how little 

 soever the choroides be touched, the impression may be easily communicated to 

 the brain ; and that the same thing may be said of the retina, there must be a 

 little channel in the optic nerve, through which the retina in its proper sub- 

 stance extends itself to this tuberosity by a continuity of fibres, which is not 

 seen ; and you are constrained to say, that there are little filaments of nerves 

 which come from the interior of the optic, and expand themselves through the 

 retina, which have this continuity; but if there were these filaments, they 

 should spread themselves through the retina, as from a centre to a circumfer- 

 ence, and should lie closer together near the optic nerve than a good way far- 

 ther in the retina, which nevertheless is not observed to be the case. 



Besides, if you thrust a pin through the thickness of the retina, you will 

 often meet with filaments; but if you look on them through a convex glass, 

 you will discover that they end in little vessels of veins and arteries, which are 



