CEREBROSPINAL NERVOUS SYSTEM. 415 
the nerves which pervade every part of the body. They also constitute the greater 
part of the brain and spinal cord. Nerve-fibres are the conducting elements of the 
nervous system which serve to bring the nerve-cells into relation both with each 
other and with the various tissues of the body. 
There are different varieties of nerve-fibres, but in all the leading and essential 
constituent is a delicate thread-like band, termed the axis cylinder. The difference 
between individual fibres depends upon the fact that in some cases the axis cylinder 
becomes invested by one or two coats, whilst in other cases it remains naked. When 
the axis cylinder is coated on the outside by a more or less thick sheath of a fatty 
substance termed myelin, it is said to be a myelinated or medullated fibre. When the 
coating of myelin is absent, the fibre is termed a non- myelinated or a non-medullated 
fibre. A second sheath—thin, delicate, and membranous, and placed externally—may 
also be present in both cases. It is termed the primitive sheath or the neurolemma. 
From a structural point of view, therefore, four different forms of ae 
nerve-fibre may be recognised :— % cylinder 
ee Jl. Naked axis cylinders. 
Meas Gunenn \2. Axis cylinders with primitive sheaths. 
halk (y 3. Primitive sheath absent. 
medullatec Peper oO een 
rimitive sheath present. 
Every nerve-fibre near its origin and as it approaches its termina- 
tion 1s unprovided with sheaths of any kind, and is simply repre- 
sented by a non-medullated naked axis cylinder. The fibres of 
the olfactory nerves afford us an example of non-medullated fibres 
furnished with a primitive sheath. 
Medullated fibres are present in greater quantity in the cerebro- 
spinal system than non-medullated fibres. Thus all the nerves 
attached to the brain and cord, with the exception of the olfactory 
and optic, are formed of medullated fibres provided with a primitive 
sheath; whilst the entire mass of the white substance of the brain 
and cord, and also the optic nerves, are formed of medullated fibres 
devoid of a primitive sheath. 
It is important to note that the distinction between the medullated 
and non-medullated fibres is not one which exists throughout all 
stages of development. As will be presently pointed out, every fibre 
is a direct outgrowth from a cell, and in the first instance none are 
provided with a medullary sheath. Indeed, it is not until about the 
fifth month of foetal life that those fibres which are to form the white 
substance of the cerebrospinal axis begin to acquire their coating of 
myelin. Further, this coating appears in the fibres of different 
strands or tracts at different periods, and a knowledge of this fact has [25 
enabled the anatomist to follow out the connexions of the tracts of ay 
Z| Primitive 
‘sheath 
fibres which compose the white matter of the brain and cord. cass 
But it may be asked: How does a nerve-fibre arise and how does Fic. 308. 
it end? Every fibre is directly continuous by one extremity with a — NeRve-ripre 
nerve-cell, whilst its opposite extremity breaks up into a number ietereri es 
of ramifications, all of which end freely in relation to another =~ , 
nerve-cell, or in relation to certain tissues of the body, as, for example, to muscle- 
fibres or to the epithelial cells of the epidermis. The length of nerve-fibres, 
therefore, varies very greatly. Some fibres are so short as merely to bring two 
neighbouring nerve-cells into relation with each other: others travel. long 
distances. Thus a fibre arising from one of the motor cells of the lower end of 
the spinal cord may, after leaving the cord, extend to the most outlying muscle 
in the sole of the foot before it reaches its destination. But even when a fibre 
does not leave the central axis a great length may be attained, and cells situated 
in the motor area of the cortex of the cerebrum may give origin to fibres which 
pass down to the lower end of the cord. 
Physiologists classify the fibres which form the nerves into two sets, afferent 
and efferent. Afferent nerve- -fibres conduct the impulse of impressions from the 
