636 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



pulling out the nerve-trunk causes only an atrophy of the central cells, and 

 not their complete disappearance. 1 



Regeneration. When the two ends of the sectioned nerve are brought 

 together under favorable conditions, the peripheral portion of the trunk may 

 be regenerated. This occurs in the following steps as described by Howell 

 and Huber. 2 



While the fragmentation and absorption of the myelin in the distal portion 

 of the cut nerves is going on, the protoplasm in the neighborhood of the 

 sheath-nuclei tends to increase. These enlarged masses of protoplasm then 

 appear as a thread of substance within the old nerve-sheath. A new sheath 

 is, however, soon formed on the protoplasmic thread, and the whole consti- 

 tutes an " embryonic fibre." The embryonic fibres lying on one side of the 

 cut unite with those on the other, union taking place in the intervening cica- 

 tricial tissue. Next the myelin appears in isolated drops, usually near the 

 nuclei, and these subsequently unite to form a continuous tube, the formation 

 of the myelin proceeding centrifugally from the wound. Then follows the 

 outgrowth of the new axis-cylinder slightly behind the organization of the 

 myelin into the tubular form. 



It must not be forgotten that the last act, the formation of the axis-cylin- 

 der, is the important event, and while the whole process of repair may require 

 many mouths, the rate at which the axis-cylinder, when started, grows out 

 from the central end may be comparatively rapid. If this explanation be 

 correct, namely that the axis-cylinder is an outgrowth from the central end, 

 then the regeneration of the neuron is in so far but a repetition of the events 

 by which it was originally formed. The development of the medullary sheath 

 in its relation to the axis is, however, different in the two cases. When first 

 regenerated, the fibres resemble normal young fibres in being small, but 

 whether they later attain the size of those which they replace has not been 

 shown. Moreover, it appears that the two functions of irritability and con- 

 ductivity do not both return at the same time. The newly formed fibres are 

 capable of conduction before they become sufficiently irritable to respond to 

 artificial stimuli directly applied to them. In the first stages of irritability, 

 also, the young fibres responded more readily to slight mechanical stimuli 

 than to induction shocks a differentiation in reaction which serves to suggest 

 the complexity of the changes involved in the re-formation of the fibres. 



Regeneration of this sort which is found in the peripheral system is not 

 known to occur in the central system, although in many ways the conditions 

 of such regeneration seem there most favorable. This fact also has its appli- 

 cation in the use of the method of degeneration for determining architectural 

 relationships ; for when once caused to degenerate, the bundles of fibres thus 

 altered can be tracked through the central system without fear that new growth- 

 changes will obscure them. 



The dorsal spinal root degenerates when the section is made between the 



1 Forel : Festschrift zur von Nageli und von Kottiker, Zurich, 1891. 



2 Journal of Physiology, 1892, vol. xiii. 



