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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



FIG. 149. Longitudinal (B) and transverse (A) 

 sections of nerve-fibres. The heavy border 

 represents the medullary sheath, which be- 

 comes thicker in the larger fibres. Human 

 sciatic nerve. X 200 diameters (modified from 

 van Gehuchten). 



A physiological significance attaches to these segments, because, as Ranvier 

 long since pointed out, it is at the nodes that various staining reagents most 



easily reach the axis-cylinder. This sug- 

 gests that normal nutritive exchanges may 

 follow the same path and thus short inter- 

 nodal segments giving rise to many nodes 

 would represent the condition most favor- 

 able to exchange between the axis and the 

 surrounding plasma. Thus far, histologi- 

 cal observation shows the more numerous 

 nodes where the physiological processes 

 are presumptively most active, and hence 

 supports the hypothesis suggested. Cases of the interpolation of new sheath- 

 ing cells to form additional segments between those originally laid down have 

 also been described. 1 



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 fibres taken from the spinal cord. 

 The conditions which there exist must be further studied before any general 

 statements concerning the medullary substance in the nerve-centres can be ven- 

 tured, 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 especially in the cerebral cortex, 

 to beyond the thirtieth year. 



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 neuron on its emergence from the cell-body, nor are ultimate branches 

 of the neuron medullated in the region of their final distribution. 



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

 that appears at the same time along the entire length of the fibre. The pro- 

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



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

 in Figure 146, the neuron often has branches near its origin, and according to 

 the observations of Flechsig 3 these may 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 physiological connections later 

 than the main stem, it would follow that their medullation should also occur 

 later, and the studies on the progressive medullation of the cerebral cortex 

 favor such a view. 



1 Vignal: Archives de Physiologic, 1883. 2 Quarterly Journal Microscopical Science, 1890. 

 s Archivfiir Anatomie und Physiologic, 1889. 



