CENTRAL NERVOUS SYSTEM. 181 



many nodes would represent the condition most favorable to exchange be- 

 tween the axis-cylinder and the surrounding plasma. Thus far, histological 



observation shows the more numerous nodes where the physiological processes 

 are presumptively most active, and hence supports the hypothesis suggested. 



In the peripheral nervous system the nerve-fibres conduct impulses before 

 they acquire their medullary sheaths : witness the activities of new-born 

 rats, in which the whole nervous system is entirely unmedullated. Moreover, 

 Langley x has reported, in the regenerating cervical sympathetic nerve, a 

 return of function, while the majority of the fibres are still without their 

 medullary sheaths. 



Medullation in Central System. — Concerning the relation of the medul- 

 lary sheath to the axis-cylinder in the central system, our information is less 

 complete. The elements which give rise to the medullary substance are not 

 known and the myelin is not enclosed in a primitive sheath. There are no 

 internodal nuclei regularly placed, yet Porter 2 has demonstrated in both the 

 frog and the rabbit the existence of nodes in some fibres taken from the spinal 

 cord. The conditions which there exist must be further studied before any 

 general statements concerning the development of the medullary substance in 

 the nerve-centres can be ventured. Yet, it is an important observation that 

 whereas medullation in the peripheral system is mainly, completed during the 

 first five years of life, the process continues in the central system, and espe- 

 cially in the cerebral cortex, to beyond the fiftieth year. 3 



Whatever views may be held concerning the capacities of a medullated 

 fibre, it is to be remembered that the medullary sheath does not cover the 

 first part of the axone on its emergence from the cell-body, nor are ultimate 

 branches of the axone medullated in the region of their final distribution. 



What has just been said applies to the main stem of the axone. As shown 

 in Fig. 70, the axone often has branches near its origin, the collaterals, and 

 according to the observations of Flechsig 4 these also become medullated. 

 Concerning the time of the medullation of these branches there are no direct 

 observations; but if it is controlled by the same conditions which appear to 

 control the process in the main stem, then, as the branches form their physio- 

 logical connections later than the main stem, it would follow thai their 

 medullation should also occur later, and the studies on the progressive medul- 

 lation of the cerebral cortex favor such a view. 



The acquisition of this sheath occurs in response to a physiological change 

 that appears at the same time along the entire length of tin 1 fibre. Tin 1 proc- 

 ess, therefore, is not a progressive one, but is practically simultaneous. 



From the observations of Ambronn and Held'' on rabbits a day or two old, 

 it appears that the efferent (motor) spinal ami cranial nerves acquire their 



1 Langley : Journal of Physiology, 1897, xxii. p. 223. 



- Quarterly Journal of Microscopical Science, 1890. 



3 Vulpius: Archiv fiir Psychiatric u/nd Nervenkrank., 1S92, lid. xxiii. 



4 Archiv fur Anatomic unci Physiologic, 1889. 



•''Ambronn and Held : Archiv fiir Anatomic and Physiologic, Anatom. A.btb.1., 1896, S. 208. 



