HISTOLOGICAL ELEMENTS 833 



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 membranes of the central nervous 

 system consist of ordinary connective tissue derived from the meso- 

 derm. The supporting framework which interpenetrates the nervous 

 substance consists of a peculiar form of tissue derived from the ecto- 

 derm, 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 neurog- 

 lia fibres, which lie close against the small glia cells. In preparations 

 impregnated by the Golgi method many of the neuroglia fibres appear 

 to be processes running out from the attenuated cell-body like the 

 arms of a microscopic crab or spider. But this is a deceptive appear- 

 ance, as has been shown by means of special methods in which the 

 neuroglia fibres are alone stained (Weigert, Huber, etc.). They 

 generally lie in close contact with, or embedded in, the protoplasm 

 of the neuroglia cells, from which they have become differentiated 

 structurally and chemically, but sometimes they may detach themselves 

 entirely from the cells and lie free in the intervening tissue. The 

 neuroglia is present in greatest abundance in the grey matter immedi- 

 ately surrounding the central canal of the cord and the ventricles of 

 the brain (the ependyma, as it is called), from which long neuroglia 

 fibres pass out radially, giving off branches on their course, and ending 

 in little knobs or enlargements attached to the pia mater. 



SECTION II. GENERAL ARRANGEMENT OF THE GREY AND WHITE 

 MATTER IN THE CENTRAL NERVOUS SYSTEM. 



(1) Around the central canal, as we have seen, a tube of grey 

 matter sheathed with white fibres is developed. This tube, from 

 optic thalamus to conus medullaris, may be conveniently referred to 

 as the central grey axis or stem, which, in the lowest vertebrates e.g., 

 fishes is much the most important part of the central nervous 

 system. 



(2) On the outer surface of the anterior portion of the neural 

 axis, but not in the part corresponding to the spinal cord, is laid 

 down a second sheet or mantle of cortical grey matter. Bet\vo-n 



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