THE CENTRAL N.ERVOUS SYSTEM 



669 



probable that the very high powers which it is so common to attribute 

 exclusively to them are, in part at least, shared with their processes. 



The grey matter, in addition to non-medullated fibres and filaments 

 representing the dendrites and such axons and collaterals as termi- 

 nate within itself, contains also, as may be seen in preparations 

 stained by Weigert's method,* great numbers of exceedingly fine 

 medullated fibres, many of which are the collaterals of fibres that 

 are passing out to the white matter. 



Only medullated nerve-fibres are met with in the white matter of 

 the cerebro-spinal axis. They are devoid of a neurilemma. In 

 diameter they vary from 2 ^ to 20 /x. In Malapterunts electricus the 

 fibre in the cord which supplies the electrical organ is of immense 

 size ; and in the anterior column of many fishes may also be seen a 

 single gigantic fibre on each side with a diameter of nearly 100 /x. 

 It cannot be said that any relation between the functions of neurons 

 and the calibre of their axons has been 

 definitely established. Manyafferent fibres, 

 it is true, are small this is notably the 

 case with the fibres of the posterior column, 

 and many motor fibres are large. But the 

 distinction can by no means be generalized, 

 for the fibres of the direct cerebellar tract 

 (p. 676), which certainly are afferent, are 

 among the largest in the spinal cord ; and 

 the vaso-motor fibres, which pass from 

 the cord by the anterior (ventral) roots 

 (Fig. 236) into the sympathetic, are smaller 

 than the fibres of the posterior column. 

 Even the motor nerve-fibres of striated 

 muscles vary considerably in diameter, 

 those of the tongue, e.g., being smaller 

 than those of the muscles of the limbs. 

 Further, the medullated fibres of the brain 

 are, without reference to function, in 

 general finer than the fibres of the cord. 

 As a rule the fibres whose course is the 

 longest are the thickest, but the rule is 

 often broken. The cause of these differences in the size of nerve- 

 fibres is quite unknown. It is more likely to be morphological than 

 physiological. 



Supporting Tissue. The protective membranes of the central 

 nervous system consist of ordinary connective tissue derived from 

 the mesoblast, the supporting framework which interpenetrates the 

 nervous substance of a peculiar form of tissue derived from the 

 epiblast, and called neuroglia. The whole cerebro-spinal axis is 

 wrapped in four concentric sheaths. Next the walls of the bony 

 hollow in which it lies is the dura mater. Next the nervous sub- 

 stance itself, following the convolutions of the brain and the fissures 



* Weigert's is a special method of staining the medullary sheath with 

 haematoxylin. 



Frc. 236. TRANSVERSE 

 SECTION OF A BUNDLE OF 

 NERVE-FIBRES FROM THE 

 ANTERIOR (VENTRAL) 

 ROOT OF THE FIRST 

 COCCYGEAL NERVE OF 

 THE CAT (DALE). 



The great difference in the 

 diameter of the fibres is well 

 shown. The small fibres are 

 vaso-motor. 



