532 



THE NEEVOUS SYSTEM. 



to form an irregular and very imperfect fibrous framework of support. The 

 neuroglia is disposed in a layer of varying thickness around the medulla, subjacent 

 to the pia mater, and is carried into the medulla so as to give a coating to both 

 sides of the various pial septa. The neuroglia is disposed also around the various 

 nerve- fibres, so that each of these may be said to lie in a canal or tunnel of this 

 substance. The nerve-fibres are all medullated, but they are not provided with 

 primitive sheaths. It is the medullary substance of the nerve-fibres which gives 

 to the white matter its opaque, milky-white appearance. When a thin transverse 

 section of the medulla is stained in carmine and examined under the microscope 

 the white matter presents the appearance of a series of closely applied circles 

 each with a dot in the centre. The dot is the transversely divided axis-cylinde] 

 of a nerve-fibre, and the dark ring which forms the circumference of the circl< 

 represents the wall of the neuroglial canal which is occupied by the fibre. Thi 

 medullary substance is very faintly seen. It presents a filmy or cloudy appearanci 



between the axis-cylinder and the neuroglial ring. 



Arrangement of the Nerve -fibres of the Whiti 

 Matter in Fasciculi or Tracts. When the whit* 

 matter of a healthy adult spinal medulla is examinee 

 the fibres which compose it are seen to vary consider 

 ably in point of size ; and although there are specia 

 places where large fibres or it may be small fibre 

 are present in greater numbers than elsewhere, yel 

 as a rule, both great and small fibres are mixed up tc' 

 gether. No conclusive evidence can be obtained in sue" 

 a spinal medulla, by any means at our disposal, of th 

 fact that the longitudinally arranged fibres are groupe 

 together in more or less definite tracts or fascicul 

 the fibres of which run a definite course and preser 

 definite connexions. Yet this is known to be tt 

 case, and the existence of these separate tracts hf 

 been proved both by embryological investigation, f{i 

 well as by the examination of the effects of injuri* 

 produced experimentally or accidentally on tl 

 nervous system in living beings. 



By the experimental method it has been shown th 

 when a nerve-fibre is severed the part which is detached fro 

 the nerve-cell from which it is an offshoot degenerates, whil 

 the part which remains connected with the nerve-cell unde 



FlG 472. TRANSVERSE SECTION g es little or no change. This is called the law of " Walleriar 



THROUGH THE WHITE MATTER OP degeneration. Thus, if in a living animal one-half of t! 

 THE MEDULLA SPINALIS, as seen spinal medulla is cut across, and after a few weeks the anim 

 through the microscope. is killed and the medulla examined, it will be seen that the 



are degenerated tracts of fibres in the white matter, both abo 



and below the plane of division ; but, still further, it will also be manifest that the tracts whi 

 are degenerated above the plane of division are not the same as those which are degenerated in t 

 part of the medulla which lies below this level The interpretation of this is obvious, 

 nerve-tracts which have degenerated above the plane of section are the offshoots of nerve-ce 

 which lie in lower segments of the medulla or in spinal ganglia below the plane of sectic 

 Severed from these nerve-cells, they undergo what is called ascending degeneration. The nen 

 tracts, on the other hand, which have degenerated in the portion of the medulla below the pla 

 of division are the axons of cells which lie at a higher level than the plane of section, either 

 higher segments of the spinal medulla or in the brain itself. Cut off from the nerve-cells fix 

 which they proceed, they present an example of descending degeneration. 



The embryological method was first employed by Flechsig, and it is often referred to 

 Flechsig's method. It is based upon the fact that nerve-fibres in the earliest stages of th; 

 development consist of naked axis-cylinders, and are not provided with medullary sheat 

 Further, the nerve-fibres of different strands assume the medullary sheaths at different peric. 

 If the foetal central nervous system is examined at different stages of its development, it is 

 comparatively easy matter to locate the different tracts of fibres by evidence of this kind. Spec- 

 ing broadly, the tracts which myelinate first are those which bring the central nervous system ii > 

 relation with the peripheral parts (skin, muscles, etc.) ; then those fibres which bind the vari( 

 segments of the central nervous system together ; next, those which connect the spinal medu - 

 with the cerebellum ; and, lastly, the tracts which connect the spinal medulla with the cereb I 

 hemispheres. The nervous apparatus for the performance of automatic movements is fu f 



