354 



DEGENERATION AND REGENERATION OF NERVES. 



the fibre, which correspond, according to Ranvier, with the intervals between the 

 medullary segments. Fifty hours after the section in the rabbit (but not till four 

 days in the dog) the protoplasmic aggregations are found here and there altogether 

 to interrupt the continuity of the medullary sheath, and they contain numerous 

 fatty granules, and sometimes droplets of myelin (fig. 416, A). About the fourth 

 day the nuclei are seen to be multiplied, but not to any great extent (C) ; and 



Fig. 416. DEGENERATION AND REGENERATION OF NERYE-FIBKES IN THE RABBIT. (Ranvier.) 



A, part of a nerve-fibre in which degeneration is commencing in consequence of section (50 hours 

 previously) of the trunk of the nerve higher up ; ??/, medullary sheath becoming broken up into drops 

 of myelin ; p, granular protoplasmic substance which is replacing the myelin ; n, nucleus, not yet 

 multiplied ; g, primitive sheath. B, another nerve-fibre in which degeneration is proceeding, the nerve 

 having been cut four days previously. This specimen is differently prepared from the others, so as to 

 exhibit the axis-cylinder (cy) also partly broken up into portions of different length, enclosed in the 

 myelin, my. C, more advanced stage of degeneration, the medullary sheath having in great measure 

 disappeared, while several nuclei (n", n") have been formed by division of the single nucleus of the 

 internode. D, commencing regeneration of a nerve-fibre. Several small nerve- fibres (t', t"), have 

 sprouted out from the enlarged cut end (b) of the nerve-fibre (t) ; a, an axis-cylinder, which has not yet 

 acquired a medullary sheath ; s, s', primitive sheath. 



the whole of the myelin after four or five days is broken up into drops, some 

 larger, some smaller. The axis-cylinder is also found to be interrupted at nume- 

 rous places, and remains only in the shape of short fibres, often curled round at 

 their broken ends, enclosed in the large drops of myelin (B). Eventually these 

 portions also may disappear. The myelin at length becomes almost entirely removed, 

 partly through the agency of leucocytes or phagocytes, until nothing remains of it 

 except a few isolated drops, which escape absorption, and all that then remains of 

 the original fibre is the primitive sheath, which is occupied by a protoplasmic mass 

 containing an increased number of nuclei. During the disappearance of the myelin 



