258 Dr. Beale Croonian Lecture. [May 11, 



and carefully described by those who have written upon the structure and 

 arrangement of nerves. The nerves distributed to a tissue or organ are 

 often represented as if they all passed straight to their terminal distribu- 

 tion, while the invariable arrangement is such as to lead to the inference 

 that, of the fibres composing a bundle of nerves, some are proceeding in a 

 direction from, and others towards the nerve-centre or peripheral part; 

 and this is observed not only in purely motor, but in purely sensitive, 

 as well as in mixed nerves. It is also found in the case of the sympathetic 

 system, and is to be demonstrated in all animals. It is, however, not pos- 

 sible to dissect the trunk of a fine nerve and render it sufficiently trans- 

 parent to display these facts, if the ordinary methods of examination be 

 adopted ; but by the plan of investigation I have fully described, the 

 arrangement may be readily demonstrated in the nerves either of the higher 

 or lower animals, although with the greatest facility in the Hyla*. Few 

 anatomical facts seem to me of more interest and importance in their 

 general bearing upon the physiology of the nervous system than that 

 above alluded to. Its constancy proves its importance, if it does not alone 

 compel us to infer that it is essential. What explanation, then, can be 

 offered of the three sets of nerve-fibres which can invariably be traced at 

 the point where a nerve-fibre comes off from a trunk passing at right angles 

 to it, as represented in these figures ? Look at it how we may, there must 

 be three sets of fibres in all cases ; and just as we find that the nerve- 

 fibres constituting the roots of the nerves divide soon after their entrance 

 in the spinal cord into bundles which pursue many different directions 

 some passing upwards towards the brain, others downwards towards the 

 lower segments of the cord, and some to the opposite side, as has been 

 well shown by the researches of Lockhart Clarke so in the case of every 

 nerve-fibre which appears to pass into or come from an adjacent nerve-trunk, 

 fibres pursue three different courses, as shown in these drawings. 



These may be afferent, efferent, and commissural; and there are fibres 

 commissural as respects different parts of the peripheral system as well as 

 of the central organ. Thus, I believe, may be explained the action of each 

 papilla as a separate organ, independently of its neighbours, or the harmo- 

 nized action of several different papillae. By the same arrangement I con- 

 sider the harmonious action of the several elementary fibres entering into 

 the formation of muscle is effected. 



As has been before observed, the large compound nerve-cords or trunks, 

 the finer bundles, and the finest constituent fibres of the pale terminal 

 nerve-fibres exhibit the same general arrangement. The remarks already 

 made with reference to the course of the fibres in the nerve-trunks and the 

 branching of the dark-bordered fibres, also apply to the finest fibres ; and 

 at the point where a fibre passes off from another at right angles, the ex- 

 istence of the three sets of fibres can be demonstrated. I would draw 



* How to work with the Microscope. Third Edition, p. 204. See also " On the 

 Branching of Nerve-trunks," &c., Archives, vol. iv. p. 127. 



