VERTEBRATED ANIMAL} 285 



this last, which is a protoplasmic substance, is the essential component 

 of the nerve-fibre, while the function of the hollow cylinder that sur- 

 rounds it, which is composed of a combination of fat and albuminous 

 matter, is simply protective. The diameter of the nerve-tubes differs in 

 different nerves; being sometimes as great as l-1500th of an inch, and as 

 small in other instances as 1-12, 000th. In many of the lower Invertebrata, 

 such as Medusce ( 523) and ComatulcB ( 546), we seem fully justified by 

 physiological evidence in regarding as Nerves certain protoplasmic fibres 

 which do not possess the characteristic structure of * nerve-tubes;' and 

 fibres destitute of the ' double contour ' are found also in certain parts of 

 the body of even the highest Vertebrates. These fibres, which are known 

 as 'gelatinous,' are considerably smaller than the preceding, and do not 

 exhibit any differentiation of parts (Fig. 476) . They are flattened, soft, 

 .and homogenous in their appearance, and contain numerous nuclear 

 particles which are brought into view by acetic acid. They can some- 

 times be seen to be continuous with the axis-cylinders of the ordinary 

 fibres, and also with the radiating prolongations of the ganglion-cells; so 

 that their nervous character, which has been questioned by some anato- 

 mists, seems established beyond doubt. 



682. The ultimate distribution of the Nerve-fibres is a subject on 

 which there has been great divergence of opinion, and which can only be 

 .successfully investigated by observers of great experience. The Author 

 believes that it may be stated as a general fact, that in both the motor 

 and the sensory nerve-tubes, as they approach their terminations in the 

 muscles and in the skin respectively, the protoplasmic axis-cylinder is 

 continued beyond its envelopes; often then breakmg-up into very minute 

 fibrillae, which inosculate with each other so as to form a network closely 

 resembling that formed by the pseudopodial threads of RMzopods (Fig. 283.) 

 Recent observers have described the fibrillae of motor nerves as terminating 

 in' motorial end-plates' seated upon or in the muscular fibres; and these 

 seem analogous to the little f islets ' of sarcodic substance, into which 

 those threads often dilate. Where the Skin is specially endowed with 

 tactile sensibility, we find a special papillary apparatus, which in the 

 skin may be readily made out in thin vertical sections treated with solu- 

 tion of soda (Fig. 477). It was formerly supposed that all the cutaneous 

 papillae are furnished with nerve-fibres, and minister to sensation: but it 

 is now known that a large proportion (at any rate) of those that are fur- 

 nished with loops of blood-vessels (Figs. 463, jo,.483), being destitute of 

 nerve-fibres, must have for their special office the production of Epider- 

 mis; whilst those which, possessing nerve-fibres, have sensory functions, 

 are usually destitute of blood-vessels. The greater part of the interior of 

 each sensory papilla (Fig. 477, c, c) of the skin is occupied by a peculiar 

 'axile body/ which seems to be merely a bundle of ordinary connective 

 tissue, whereon the nerve-fibre appears to terminate. The nerve-fibres arc 

 more readily seen, however, in the ' fungiform ' papillae of the Tongue, 

 to each of which several of them proceed; these bodies, which are very 

 transparent, may be well seen by snipping-off minute portions of the 

 tongue of the Frog; or by snipping-off the papillae themselves from the 

 surface of the living Human tongue, which can be readily done by a dex- 

 terous use of the curved scissors, with no more pain than the prick of a 

 pin would give. The transparence of these papillae also is increased by 

 treating them with a weak solution of soda. Nerve-fibres have also been 

 found to terminate on sensory surfaces in minute ( end-bulbs ' of spher- 

 oidal shape and about l-600th of an inch in diameter; each of them being 



