732 EXAMINATION OF NERVE-SUBSTANCE. 



ing Nerve-fibres, have Sensory functions, are usually destitute of 

 Blood-vessels. The greater part of the interior of each Sensory 

 papilla (Fig. 392, 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 are 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 dexterous use of the curved scissors, 

 with no more pain than the prick of a pin would give. The trans- 

 parence of any of these Papilla? is increased by treating them 

 with a weak solution of Soda. 



573. For the sake of obtaining a general acquaintance with the 

 Microscopic characters of these principal forms of Nerve-substance, 

 it is best to have recourse to minute Nerves and Ganglia. The 

 small Nerves which are found between the Skin and the Muscles of 

 the back of the Frog, and which become apparent when the former 

 is being stripped-off, are extremely suitable for this purpose ; but 

 they are best seen in the Hyla or 'Tree-Frog,' which is recom- 

 mended by Dr. Beale as being much superior to the Common Frog 

 for the general purposes of minute Histological investigation. If 

 it be wished to examine the natural appearance of the Nerve-fibres, 

 no other fluid should be used than a little Blood-serum ; but if 

 they be treated with strong Acetic acid, a contraction of their tubes 

 takes-place, by which the axis-cylinder is forced-out from their 

 cut extremities, so as to be made more apparent than it can be in 

 any other way. On the other hand, by immersion of the tissue in 

 a dilute solution of Chromic acid (about one part of the solid 

 crystals to two hundred of water), the Nerve -fibres are rendered 

 firmer and more distinct. Again, the Axis-cylinders are brought 

 into distinct view by the Staining-process (§145), being dyed much 

 more quickly than their envelopes ; and they may thus be readily 

 made-out by reflected light, in transverse sections of Nerves that 

 have been thus treated.* The Gelatinous fibres are found in the 

 greatest abundance in the Sympathetic nerves ; and their characters 

 may be best studied in the smaller branches of that system. — So, 

 for the examination of the Ganglionic cells, and of their relation 

 to the Nerve-tubes, it is better to take some minute Ganglion as a 

 whole (such as one of the Sympathetic ganglia of the Frog, Mouse, 

 or other small animal), than to dissect the larger ganglionic masses, 

 whose structure can only be successfully studied by such as are 

 proficient in this kind of investigation. The Nerves of the orbit of 

 the Eyes of Fishes, with the Ophthalmic Ganglion and its branches, 

 which may be very readily got-at in the Skate, and of which 



* See Prof. Lister's ' Observations on the Structure of Nerve-fibres,' 

 in " Quart. Journ. of Microsc. Science," Vol. viii. (I860), p. 29. 



