CENTRAL NERVOUS SYSTEM. 615 



irritability and conductivity of nerve-fibres l it was found that certain nerve- 

 fibres, notably the vaso-constrictor fibres and the sweat-fibres in the sciatic nerve 

 of the cat, when they were subjected to a faradic current continued for several 

 minutes, lost their irritability, completely or in part, at the point of stimula- 

 tion. This " stimulation fatigue " is not known to be produced in nerves 

 which are unquestionably medullated. It does occur where the nerves are 

 unmedullated, but it also occurs where the absence of medullation has not been 

 proved, and hence cannot be put forward as a differential character distinguish- 

 ing these two sorts of nerves. The medullated neurons are in their early history 

 unmedullated, and only later acquire this sheath, so that medullation might be 

 taken to represent a final step in the highest development of the nerve-cell. The 

 fact that certain groups of fibres are not functional till after they are medullated 

 hardly bears on the question, for the following reason : Until a group of fibres 

 has established a physiological connection with the tissues which it is to control, 

 it cannot be expected to influence them, and it has yet to be shown that the 

 appearance of functional activity and the beginnings of medullation are not 

 both of them the result of such growth-changes at the distal end of the axis- 

 cylinder. The changes involved in establishing physiological connections are 

 those by which the tips of the branches formed by the neuron of one cell come 

 into such relation with other branches of a second cell or some non-nervous 

 tissue that the nerve-impulse can pass between them. At the same time the 

 non-medullated neurons establish connections with the tissues controlled by 

 them just as well as do those which are to be medullated, but why one goes 

 on to the acquisition of- the sheath and the other remains without it, is not 

 explained. Neither is it known how far one of these forms may replace the 

 other, although, it is not improbable that the proportions of medullated and 

 unmedullated fibres in different persons may be very unlike. 



Growth of Medullary Sheath. Whatever may be the significance of the 

 medullary sheath it is usually formed before the nerve-element as a whole has 

 attained its full size. In the peripheral system it depends on the presence of 

 cells which envelop the axis-cylinder, forming a tube about it. Each ensheath- 

 ing cell is physiologically controlled by a nucleus which becomes situated about 

 midway between its extremities. The cell-substance is largely transformed into 

 myelin, and the line of junction between two of these sheathing cells forms a 

 node of the nerve-fibre. In the sheath of a growing nerve-cell at least two 

 changes are clearly marked : As the axis increases in diameter the medul- 

 lary sheath becomes thicker. The change is such that in the peripheral 

 system the areas of the axis-cylinder and of the medullary sheath as shown 

 in cross sections remain nearly equal (Fig. 149). On the other hand the length 

 of the internodal segments tends to increase with an increase in the diameter 

 of the nerve-fibre, and for nerves of the same diameter it is less in man than 

 in the lower mammals. In a given fibre the segments are shorter at the extreme 

 peripheral end (Key and Ketzius). In the young fibres, also, they are shorter 

 and increase in length with age. 



1 Howell, Budgett, and Leonard : Journal of Physiology, 1894, vol. xvi. 



