CLARKE, ON NERVE-FIBRE. 69 



inwards and outwards. Those which run outwards become 

 connected with the sheaths of the neighbouring nerve-fibres ; 

 while those running inwards come into connexion with the 

 medullary sheath (white substance of Schwann) and the 

 axis-cylinder. 



2. The white substance of Schwann consists of a number- 

 less multitude of the finest fibres, which take the most 

 varied course, but run mostly in oblique or in transverse 

 directions, and establish a communication between the 

 external sheath and the axis-cylinder. 



3. The axis-cylinder consists of three layers, one within 

 the other, each of which sends out, in horizontal and oblique 

 directions, numberless ramifications, which cross the space, 

 and so constitute the fibres of the white substance, and 

 come into connexion with the so-called (external) sheath. 

 Whether the inner or central portion of the axis-cylinder is 

 hollow, or composed of the finest fibrils, is uncertain, but the 

 latter is more probable. 



Besides the parts just described, each primitive nerve-fibre 

 contains an oleaginous fluid, which hitherto was considered 

 to be included between the external sheath and the axis- 

 cylinder, as in a hollow space or vessel. But this fluid is, 

 in all probability, contained in the finest tubules of the white 

 substance, as well as in those of the axis-cylinder and the 

 sheath.* 



The " elementary tubules" of which each primitive nerve- 

 fibre is said to be composed are exceedingly small, varying 

 from the TT ^ oth to the ¥ ^ -oth of a line in diameter. 



Now it would be very difficult to account for the produc- 

 tion of such elementary fibres as these under the influence 

 of chromic acid, however disinclined we might be to acknow- 

 ledge them as parts of the normal structure, until further 

 proofs were adduced ; and therefore, if we really admit 

 their existence — whether they be tubules or only simple 

 fibres, — and consider, atthe same time, the prolonged attention 

 of Stilling to the subject, as well as the facts brought to light 

 by Remak and others, we ought, I think, with Kolliker, to 

 be cautious in pronouncing a decided judgment. But the 

 question may be completely set at rest, for, as will presently 

 be seen, these supposed elementary tubules, or fibres, have 

 no actual existence whatever, — the appearances from which 

 they have been inferred resulting solely from corrugations, 

 ridges, or folds produced in the white substance by the 

 action of the chromic acid. 



* 'Neue Untersuch.,' ite Lieferimg, p. 70S. 



