758 



A MANUAL OF PHYSIOLOGY 



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 ju to' 20 p. In Malapterurus 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 /*. 

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

 and the calibre of their axons has been definitely established. Many 

 afferent 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. 764), which certainly are afferent, 

 are amongst the largest in the spinal cord ; and the vaso-motor 

 fibres, which pass from the cord by the anterior (ventral) roots 

 (Fig. 312) 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. 

 For example, the average diameter of the 

 fibres going to the thigh of the frog is 

 greater than that of the fibres going to 

 the lower part of the limb (Dunn). 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 

 ANTERIOR (VENTRAL) ROOT membranes of the central nervous system 

 OF THE FIRST COCCYGEAL consist of ordinary connective tissue de- 

 NERVE OF THE CAT (DALE). rived from the me soderm. The support- 

 The great difference in the ing framework which interpenetrates the 

 diameter of the fibres is well nervous substance consists of a peculiar 



form of tissue derived from the ectoderm, 

 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 substance itself, following the 

 convolutions of the brain and the fissures of the cord, and giving 

 off bloodvessels to both, is the pia mater. Between the dura and 

 the pia, separated from the latter by a jacket of cerebro-spinal 

 fluid, is the double layer of the arachnoid. The comparatively 

 coarse septa that run into the nervous substance as if coming off 

 from the pia mater are the main beams in the scaffolding of 

 non - nervous material with which that substance is interwoven, 

 and by which it is supported. The interstices are filled in by a 

 thick-set feltwork of interlacing neuroglia .fibres, which lie close 

 against the small glia cells, but according to some authorities are, 

 in the adult at least, perfectly distinct from them, although origi- 

 nally formed from the cells. In preparations impregnated by the 

 Golgi method many of the neuroglia fibres appear to be processes 



FIG. 312. TR A N s VE RS E 

 SECTION OF A BUNDLE OF 

 NERVE-FIBRES FROM THE 



shown. The small fibres are 

 vaso-motor. 



