268 



pulpy, but not alike in all, in consistence and form ; and it is to these va- 

 rieties of arrangement and structure, that the varieties of sensation in 

 the different organs are to be referred. 



One may say that there exists, in the organs of sense, a certain relation, 

 between the softness of the nervous extremity, and the nature of the bo- 

 dies which produce an impression upon it. Thus, the almost fluid state 

 of the retina, bears an evident relation to the subtilty of light. The con- 

 tact of this fluid could not produce a sufficient impression, unless the 

 sentient part were capable of being set in motion by the slightest im- 

 pression. The portio mollis of the seventh pair, wholly deprived of its 

 solid covering, and reduced to its medullary pulp, readily partakes in the 

 sonorous motions transmitted to it by the fluid, in the midst of which its 

 filaments are immersed. The nerves of smell arid of taste are more ex- 

 posed, than the nervous papillae of the skin employed in receiving the 

 impressions produced by the coarser properties of bodies, See. 



From their origin, the nerves ascend towards the medulla oblongata 

 and the spinal marrow, in a line nearly straight, and seldom tortuous, as 

 most of the vessels. When they have reached these parts, they termi- 

 nate in them and are lost, in their substance, as will be mentioned, in 

 speaking of the structure of these nervous cords. 



CXXXVIII. Every nerve is formed of a greater number of filaments, 

 extremely delicate, and which have two extremities, the one in the brain 

 and the other from the part in which they terminate, or from which they 

 originate. Each of these nervous fibres, however minute, is composed 

 of a membranous tube, which is a production of the pia mater. Within 

 the parietes of this tube, there are distributed innumerable vessels of 

 extreme minuteness: it is filled with a whitish marrow, a kind of pulp, 

 which Reil states he insulated from the small canal containing it, by con- 

 creting it, by means of the nitric acid, which dissolves the membranous 

 sheath, and leaves uncovered the medullary pulp forming the essential 

 part or basis of the nervous filament. The same physiologist discovered, 

 by a different process, the internal structure of each nervous fibrilla; he 

 dissolved the whitish or pulpy part, by a long continued solution in an 

 alkaline ley, and he succeeded thus in separating it from the membranous 

 tube which enclosed it and which was emptied. The membranous sheath 

 is of cellular structure, and is remarkable only by its consistence and by 

 the very considerable number of vessels of all kinds that are distributed 

 to its parietes; it ceases to cover the nerves near their two extremities, 

 and protects them only along their course. 



Each nervous fibre, thuss formed of two very distinct parts, joins other 

 fibres of a perfectly similar structure, to form a nervous filament enve- 

 loped in a common sheath of cellular tissue. These filaments by their 

 union, form small ramifications, and those progressively larger branches, 

 and lastly, trunks w? apped in a. common covering of cellular tissue; then 

 other envelopes to each fasiculus of filaments, and lastly, a sheath to each 

 individual filament. When nervous cords are of a certain size, veins and 

 arteries of a pretty considerable calibre, may be seen to insinuate them- 

 selves between the bundles of fibres of which they are composed; these 

 vessels then divide, after penetrating among them, and furnish the capil- 

 lary ramifications which are distributed to the parietes of the sheath 

 common to each filament. These small vessels, according to Reil, allow 

 the nervous substance, to exhale into each membranous tube; this, like- 

 wise, becomes the secretory organ of the medulla with which it is filled. 



