TOL. LXXX1X.] PHILOSOPHICAL TRANSACTIONS. 433 



visible: the entrance of the optic nerve was a very white spot, which seemed to be 

 opaque; but, when small pieces of coloured paper were alternately placed between 

 the outside of the eye, and the bottom of the basin, their colour was distinctly 

 seen in the cavity of the eye, through the substance of the optic nerve ; so that, 

 at this part, the internal structure of the nerve has a degree of transparency. 

 This appeared to be a newly-discovered fact; and, to ascertain whether it was 

 really so, the works of several physiological writers were consulted, but nothing 

 was found which gave an idea that their authors had the smallest knowledge of it. 

 This semi-transparent state of the internal parts of the optic nerve, while recent, 

 led naturally to the examination of its substance, by means of magnifying glasses; 

 and notwithstanding the failure of so many men of superior abilities, in this intri- 

 cate inquiry, it held out the hope of meeting with some success. 



The principal theories which have been formed respecting the structure of nerves, 

 which have been taken notice of by Fontana, as they all differ from the observa- 

 tions which will be stated in the present paper, it may not be improper to mention 

 the heads of each of them, so as to bring into one point of view, all the know- 

 ledge that has been acquired on the subject. Torre found the medullary substance 

 of the brain, spinal marrow, and nerves, to be a mass of transparent globules, 

 swimming in a transparent fluid. When the parts were magnified 1000 times, 

 the globules appeared largest in the brain, and smaller in the spinal marrow; they 

 had no regular order: but in the nerves the globules were placed in lines, so as to 

 give the appearance of fibres. In examining the optic nerve, the parts were mag- 

 nified 120 times. Prochaska considered the nerves to be composed of globules, 

 united by a transparent elastic cellular membrane, and disposed in straight lines, 

 resembling fibres. Fontana found the primitive structure of nerves to consist of 

 transparent cylinders, which, when united, formed the nerve: the manner of their 

 being disposed is not mentioned. The objects were magnified 700 times, to show 

 this appearance. Dr. Monro considered the nerves as made up of spiral fibres; 

 but afterwards found that what he had described was entirely an optical deception. 

 In his last work, he says, " The optic nerves have, in their whole course, less 

 appearance of a fibrous structure than perhaps any other pair of nerves in the 

 human body." Other authors may have written on this subject, and may have 

 made observations on the structure of nerves, but want of leisure must be an 

 excuse for my not having come to a knowledge of them. 



It is scarcely necessary to mention, that parts of an animal body are not fitted 

 for being examined by glasses of a great magnifying power; and wherever they are 

 shown 100 times larger than the natural size, no dependence can be placed on 

 their appearance. In making the following microscopical experiments on the 

 internal structure of the optic nerve, great care was taken to avoid the errors of 

 former inquirers. The microscope used was a single one; the focal length of the 

 lens was about tVo°^ an inch, so that the object was magnified about 23 times; 

 and, that the results of the experiments might be as free from optical deceptions 



VOL. XVIII. 3 K 



