1865.] Supposed Terminations of Nerve- fibres. 251 



THE ESSENTIAL STRUCTURE OF A NERVOUS MECHANISM 

 CONSIDERED. 



Of the supposed terminations of the dark-bordered nerve-fibres, and of the 

 probable existence of nerve-circuits. 



It will have been remarked that Continental observers are unanimous in 

 representing the dark-bordered nerve-fibre as passing to its destination 

 unaccompanied by any other fibre whatever. In some drawings it is re- 

 presented as terminating in a short fine fibre, which is regarded as the 

 prolongation of the axis-cylinder ; in others, this fine fibre is represented 

 as bifurcating so as to form two very fine fibres. Some observers consider 

 that the "axis-cylinder" spreads out to form a band which is more or 

 less convoluted, but terminal, forming an " end-organ," while others hold 

 that the fibre gradually ceases or loses itself in the surrounding tissue. 



But while there are these minor differences, all agree in the opinion that 

 the nerve-fibre passes alone to its ultimate "end." On the other hand, I 

 have endeavoured to show that at least one fine nerve-fibre accompanies the 

 dark-bordered fibre to its ultimate destination (page 241) as represented in 

 these figures, and that this fine nerve- fibre is a constant structure of great 

 importance. It was first fully described by me in papers in the 'Archives 

 of Medicine ' on the frog's bladder; but its existence was referred to in 

 my paper in the Philosophical Transactions for 1860, and its arrangement 

 investigated in that published in the Philosophical Transactions for 1862. 



Some surprise may be felt that Continental observers have not specially 

 noticed the fine fibre accompanying the dark-bordered fibre, or referred 

 to my statements concerning it ; but as neither the fine fibre, nor indeed the 

 finest part of the dark-bordered fibre, can be seen in specimens examined 

 in aqueous fluids, it was scarcely to be expected that the facts I have 

 described should have been verified in Germany. I therefore beg to direct 

 the attention of anatomists and physiologists to the drawings to which I 

 now point, and to my specimens. 



The fine fibre accompanies the dark-bordered fibres distributed to the 

 tissues of the frog generally, but it is more easily demonstrated in relation 

 with the nerves distributed to the bladder, to the mucous membrane of the 

 palate, the skin, and the connective tissue about the heart and lungs, than 

 with those of striped muscle ; it is, however, so frequently seen in the case of 

 muscular nerves, especially in the mylohyoid of the Hyla, that I believe 

 it is invariably present, though it cannot be demonstrated in all cases. 

 Not only is the structure of the fibre very delicate, but it is often obscured 

 by the dark-bordered fibre which it accompanies. 



Now, if a single fibre passed at once to its destination, as Continental 

 observers suppose, it is obvious that the arrangement of the nerves in 

 muscle must be different in principle to their arrangement in the cornea, 

 for example, where it is admitted no " end-fibres " can be detected. But, on 

 the other hand, as at least two fibres, and usually several, pass together to 



