520 PHILOSOPHICAL TRANSACTIONS. [aNNO IJQS. 



comparison of it with the original nerve ; or a cautious attention to the function 

 of that nerve, by which we see the loss of it from the division, and the return 

 of it from tlie re-union of the divided parts. Those who have subjected this 

 matter to the test of experiment, have made their appeal to the first criterion ; 

 and have either affirmed or denied the reproduction, according as they thought 

 the new formed part either agreed with or differed from the original nerve. 



This criterion certainly supposes, that anatomy is fully competent to determine, 

 what is the precise structure of nerves, what are the nature and characters of 

 ultimate nervous fibres, and by what mechanism or power they execute their 

 allotted function. It supposes also, that anatomists are perfectly agreed on this 

 matter ; and that those who make their appeal to anatomy, have admitted a 

 common standard of comparison, by which they allow their experiments to be 

 judged; but no position is more remote from fact. It is sufficient to say, that 

 some think ultimate nervous fibres are constructed to act by tremors, while 

 others believe them to be hollow tubes. Nor is the difference of opinion less, 

 respecting the appearances which they exhibit on being viewed by a microscope. 

 One eminent physiologist* observes, that the ultimate nervous fibres are " ser- 

 pentine and convoluted, very much resembling the winding of the seminal ducts 

 in the testicle, or epididymis :" but having extended his microscopical observa- 

 tions to other parts, he finds a similar disposition of fibre ; nay even neutral 

 salts, in a state of crystallization, and metals, when microscopically examined, 

 have convoluted fibrous appearances, corresponding with those of nerves. An- 

 other ingenious inquirer, -|- having subjected the nerves to microscopic examina- 

 tion, thought at one time that their fibres were composed of cylinders, with 

 bands twined around them, in a spiral direction ; but subsequent examinations 

 convinced him, that this appearance had its origin in an optical deception, and 

 that their true direction was that of " parallel winding fibres." I have not yet 

 heard whether a 3d examination has rectified the errors of the 2 former. 



As it appears then, that microscopical observers neither agree with each other 

 on this subject, nor with themselves, I think it fair to conclude, that ocular 

 inspection cannot be admitted as a fair appeal, from which we can (;letermine 

 whether the substance which unites the extremities of divided nerves is of the 

 same nature as the original nerve. Dr. Arnemann, of Gottingen, who has 

 written ex professo on the reproduction of nerves, denies positively, from anato- 

 mical examination, that the new formed substance is of the nature of nerve ; 

 and on being shown the result of some of my experiments, he declared at the 

 first glance of the eye, " that tiie medium of union did not possess the characters 

 of nerve ;" and further, " that the true nervous substance is never reproduced." 

 But he had already prejudged the matter. On the other hand, I am persuaded 



* Dr. Monro, f Fontana. — Orig. 



