346 OP THE PRIMARY TISSUES OF THE HUMAN BODY. 



(provided the degeneration have not proceeded too far), when light is again 

 allowed to act upon the retina. For it may be observed that, when an eye has 

 been from any cause withdrawn for a long period from its normal state of acti- 

 vity, and is again placed in a condition to be employed (as when it has been 

 turned inwards by extreme Strabismus, and has been brought by operation into 

 the correct axis; or when closure of the pupil, or the extension of an opacity over 

 the central part of the cornea, has been remedied by making an artificial pupil), 

 the recovery of good sight is by no means immediate, but often requires a con- 

 siderable lapse of time, like the recovery of muscular power in a limb which has 

 become atrophied from paralysis. Various experimental results confirm this 

 view. Thus it was found by Grunther and Schbn, that when a nerve is divided, 

 the peripheral portion exhibits obvious degenerative changes within the space of 

 a week; 1 and these changes have been shown by Nasse to be still more decided 

 some months afterwards. 3 The most minute and interesting researches yet 

 made upon this point, however, are those of Dr. Aug. Waller; 3 who has taken 

 for the subject of his investigations the alterations produced in the nerves of the 

 Frog's tongue, the papillae of which are supplied by the glosso-pharyngeal. 

 Within three or four days after the division of the trunk of this nerve on one 

 side, a difference begins to manifest itself between the fibrils contained in the 

 papillae which it supplies, and those of the sound side ; for a slightly turbid or 

 coagulated appearance is seen in the contents of the tubes, which no longer ap- 

 pear to fill them ; and the difference is still more marked twenty-four hours after 

 death. About five or six days after the section, the alteration of the nerve- 

 tubes, produced by the coagulation of their medullary contents, becomes very 

 obvious ; the diameter of the altered tubes is about a fourth less than that of the 

 sound ones; and in many parts, the membranous tubules cannot be distinguished, 

 the coagulated masses appearing to be completely disjointed from one another. 

 On the eighth and ninth days the coagulated particles. become still more discon- 

 nected, and are in parts removed by absorption ; on the tenth day, the particles 

 begin to assume a granular texture ; and by about the twentieth day they are 

 completely reduced to the granular state, the presence of the nervous element 

 being only indicated by rows of numerous dark granules, arranged like the beads 

 of a necklace. The resistance of these granular bodies to chemical agents has been 

 found by Dr. Waller to be most remarkable; they remain unaffected by acids, 

 alkalies, and ether; and he has seen them apparently unaltered after a lapse of 

 more than five months. As all the nutritive changes in cold-blooded animals 

 go on with greater rapidity at a high than at a low temperature, so should we 

 expect that the degenerative changes would likewise, upon the principles for- 

 merly laid down ( 114); and it is an interesting confirmation of this view to 

 find, that Dr. Waller has noticed that the alterations consequent upon section 

 of the nerves, make themselves apparent in frogs much earlier in summer than 

 they do in winter. 



348. The first development of the Nerve-tubes appears to take place, like that 

 of Muscular fibre, by the coalescence of a number of primary cells into a con- 

 tinuous tube; for although the primary nervous cell has not yet been made out 

 with precision, the nuclei of what seem to be the original cells may frequently 

 be seen in the fully-formed tube, lying between their membranous walls and the 

 white substance of Schwann (Fig. 106, c). When first a nerve-fibre can be recog- 

 nized as such, it has a strong resemblance to the t( gelatinous" fibres of the 

 sympathetic trunks; being a cord of small diameter, without any clear distinc- 

 tion between the tube and its contents, of granular consistence, and having 

 nuclei at no great distance from each other. The substance of the fibre, at this 

 period, seems to correspond with the axis-band of the fully-formed nerve-tube; 



1 "Miiller's Archiv.," 1840, p. 276. 2 Op. cit., 1839, p. 409. 



3 "Philosophical Transactions," 1850, p. 423, et seq. 



