MICROSCOPIC CHARACTERS 01' NEKVB-SUBSTANCE. 615 



tubular sheath, for its complete isolation. The contents of the 

 membranous envelope are very soft, yielding to shght pressure ; 

 and they are so quickly altered by the contact of water or of any 

 liquids that are foreign to their nature, that their characters can 

 only be propei'ly judged of when they are quite fresh. Besides 

 the proper tubular fibres, however, there are others, known as 

 "gelatinous," which are considerably smaller than the preced- 

 ing, and do not exhibit any dift'erentiation of parts. They are 

 flattened, soft, and homogeneous in their appearance, and con- 

 tain numerous nuclear particles, which are brought into view by 

 acetic acid. They can sometimes be seen to be continuous with 

 the axis-cylinders of the ordinary fibres, and also with the radiat- 

 ing prolongations of the vesicles ; so that their nervous character, 

 which has been doubted by some anatomists, seems established 

 beyond doubt. The ultimate distribution of the nerve-fibres may 

 be readily traced in thin vertical sections of the skin, treated 

 with solution of soda. It was formerly supposed that all its 

 papillge are furnished with nerve-fibres, and minister to sensa- 

 tion ; but it is now known that a large proportion (at any rate) 

 of those furnished with loops of bloodvessels (Fig. 329, d), being 

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

 production of the epidermis; whilst those which, possessing 

 nerve-fibres, have sensory functions, are usually destitute of 

 bloodvessels. The greater part of the interior of each sensory 

 papilla of the skin, is occupied by a peculiar " axile body," 

 which seems to be merely a bundle of ordinary fibrous 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 

 ofi" minute portions of the tongue of the Frog ; or by snipping 

 oft' 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 transparency of any of these papillifi is in- 

 creased, by treating them with a solution of soda. 



430. 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 gan- 

 glia. The small nerves which are found between the skin and 

 the muscles of the back of the Frog, and which become appa- 

 rent when the former is being stripped off, are extremely suita- 

 ble for this purpose ; and 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 Avay. ^ 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 



