CENTRAL NERVOUS SYSTEM. 199 



It is sometimes stated that degeneration takes place in the direction of 

 the nerve-impulse. In a general way this is true, since the impulses usually 

 travel from the cell-body along the axone. In the case of the fibres arising 

 from the cells of the spinal ganglion it is not true, since the section at the 

 distal side of the ganglion causes degeneration away from the spinal cord, 

 while that on the proximal side of the ganglion causes degeneration toward 

 the spinal cord ; yet in both axones the impulse is in the same direction — 

 namely, toward the cord (see Fig. 85). 



Degeneration of the Cell-body. — It was discovered by von Gudden 1 

 that when the nerves of young animals are pulled away from their attach- 

 ment with the central system, they most frequently break just at the point 

 where they emerge from the cord or brain axis. When an efferent nerve is 

 thus broken, in animals just born or very young, the remaining portion — i. e. } 

 the cell-bodies with so much of their axones as lie within the central system — 

 may atrophy to complete disappearance. 



The bearing of such a fact is very direct. If in man there is reason to 

 think that an injury was suffered during fetal life, there is a possibility that 

 the injury may not only have prevented the further development of the cells 

 involved, but may also have caused the complete destruction of some of them, 

 in which case, of course, the architecture of the damaged region is necessarily 

 abnormal. 



Such complete disappearance as the result of early injury has not been 

 shown for cells which lie entirely within the central system. Those forming 

 the spinal ganglia may die, however, after interruption of the axones, even 

 when the animal is mature (van Gehuehten). In the case of those central 

 cells which form the sensory nuclei, like the sensory nucleus of the fifth 

 nerve, or of the vagus, pulling out the nerve-trunk formed by the axones of 

 the afferent ganglion cells, causes only an atrophy of the central cells, and 

 not their complete disappearance. 2 



Regeneration. — When the two ends of the sectioned nerve arc brought 

 together under favorable conditions, the peripheral portion may be regener- 

 ated. This regeneration occurs only in axones possessing a nucleated (inedul- 

 lated or unmedullated) sheath, or in the anatomical prolongations of these, 

 such as the dorsal root-fibres which penetrate the spinal cord. 3 In the 

 typical medullated peripheral nerve this process occurs in the following 

 steps as described by Howell and Huber : ' 



While the fragmentation and absorption of (he 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- 



1 Archivfur PsyckUUrie, 1S70, Bd. ii. 



2 Forel : Festschrift zux run Nageli und run Kolliker, Zurich, 1891. 



:t Raer, Dawson, and Marshall : Journal of Experimental Medicine, L899, vol iv. No. 1. 



4 Journal of Physiology, 1892, vol. xiii. 



