a 
COMPONENT PARTS OF WHITE MATTER OF SPINAL CORD. 431 
COMPONENT PARTS OF THE WHITE MATTER OF THE CorD. 
The white matter of the cord is composed of medullated nerve-fibres embedded 
in neuroglia. The fibres, for the most part, pursue a longitudinal course ; and from 
the deep surface of the pia mater which surrounds the 
cord fibrous septa or partitions are carried in along \\\\\js 3-2 
vertical planes between the fibres, so as to form an i ) BaCare 
irregular and very imperfect fibrous framework of 
support. The neurogha is disposed in a layer of 
varying thickness around the cord, subjacent to the | 
pia mater, and is carried into the cord so as to give a 
coating to both sides of the various pial septa. The 
neuroglia also is disposed around the various nerve- |} | 
fibres, so that each of these may be said to he ina (|i 
canal or tunnel of this substance. The nerve-fibres | '/ 
are all medullated, but they are not provided with | WNVeas 
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 trans- 
verse section of the cord 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 cylinder of a ne1ve-fibre, and 
. : S . Neg 
the dark ring which forms the circumference of the = “ 
ln 2 Y 
circle represents the wall of the neuroglial canal which — |" Mh Pn 
is occupied by the fibre. The medullary substance is * 
very faintly seen. It presents a filmy or cloudy fg Pee 
appearance between the axis cylinder and the Fic. 321.—Traysverse Secrion 
neuroghal ring THROUGH THE WHITE MATTER OF 
fo) co fe 
Arrangement of the Nerve-fibres of the White En iS Seo aa 
Matter in Strands or Tracts.— When the white 
matter of a healthy adult cord is examined the fibres which compose it are seen 
to vary considerably in point of size; and although there are special places where 
large fibres —or it may be small fibres—are present in greater numbers than 
elsewhere, yet as a rule both great and small fibres are mixed up together. 
Absolutely no evidence can be obtained in such a cord, by any means at our 
disposal, of the fact that the longitudinally-arranged fibres are grouped together 
in more or less definite tracts or strands, the fibres of which run a definite course 
and present definite connexions. Yet we know this to be the case, and the 
existence of these separate tracts has been proved both by physiological and by 
embryological investigation. 
The physiological evidence depends on the fact that when a nerve-fibre is severed the part 
which is detached from the nerve-cell from which it is an offshoot degenerates, whilst the part 
which remains connected with the nerve-cell undergoes little or no change. This is called the 
law of “ Wallerian” degeneration. Thus, if in a living animal one-half of the cord be cut across, 
and after a few weeks the animal be killed and the cord examined, it will be seen that there 
are degenerated tracts of fibres in the white matter, both above and below the plane of division ; 
but, still further, it will also be manifest that the tracts which are degenerated above the plane 
of division are not the same as those which are degenerated in the part of the cord which lies 
below this level. The interpretation of this is obvious. The nerve-tracts which have degenerated 
above the plane of section are the offshoots of nerve-cells which lie in lower segments of the cord 
or in spinal ganglia below the plane of section. Severed from these nerve-cells, they undergo 
what is called ascending degeneration. The nerve-tracts, on the other hand, which have 
degenerated in the portion of the cord below the plane of division are the axons of cells which 
lie at a higher level than the plane of section, either in higher segments of the cord or in the 
brain itself. Cut off from the nerve-cells from which they proceed, they present an example of 
descending degeneration. 
The embryological evidence we owe to Flechsig, and it is no less satisfactory. It depends 
upon the fact that nerve-fibres in the earliest stages of their development consist of naked axis 
cylinders, and are not provided with medullary sheaths. Further, the nerve-fibres of different 
strands assume the medullary sheaths at different periods). By examining the fetal cord at 
