420 THE NERVOUS SYSTEM. 
peripheral division, which becomes connected with a peripheral part, may pro- 
visionally be regarded as a dendrite. The central fibres from the ganglionic cells 
in the region of the cord form the dorsal or posterior roots of the. spinal nerves. 
These roots have thus an origin outside the cord, and grow into its substance in 
the process of development in the same manner that the roots of a plant strike 
into the soil. 
4, The cerebrospinal neurons are derived from the germinal cells in the wall 
of the early neural tube. Certain of these furnish efferent nerve-fibres, which 
issue from the cord in separate bundles termed the anterior or ventral roots of the 
spinal nerves. In the case of the cranial nerves, however, with the exception of the 
trigeminal and facial nerves, the efferent fibres are not thus separated from the 
ee fibres at their attachment to the brain. 
The brain and cord when studied by the naked eye are seen to be composed 
of ae matter and gray matter. The white matter forms very nearly two-thirds 
of the entire cerebrospinal axis. It is composed of medullated nerve - fibres 
embedded in neuroghal tissue. The gray matter is composed of nerve-cells with 
their dendrites and axons. Some of the axons are in the form of naked axis 
cylinders, whilst others have a coating of medulla. Intimately intermixed with 
these parts is the neuroglia, which isolates them more or less completely from 
each other. 
SPINAL CORD. 
The spinal cord is that part of the cerebrospinal axis which occupies the upper 
two-thirds of the spinal canal of the vertebral column. It is an elongated cylin- 
drical structure, slightly flattened in front and behind, which extends from the 
margin of the foramen magnum to the level of the lower border of the body of 
the first lumbar vertebra or to the upper border of the body of the second lumbar 
vertebra. Its average length in the male is 45 cm. 
and in the female 45 cm. 
Cerebral ° oo “yaks 5 
hemisphere A considerable amount of variation within certain 
1—Mesencephalon limits (viz. the mid-point of the body of the last dorsal 
i vertebra and the upper border of the body of the 
third lumbar vertebra) is observed in different in- 
Fourth ventricle dividuals as to the precise level at which the spinal 
Medulla cord ends inferiorly, and in the female there would 
appear to be a tendency for the cord to reach a slightly 
Cervical swelling lower point in the canal than in the male. Further 
of the cord the relation presented by the spinal cord to the 
vertebral column differs in a marked degree in the 
foetus and infant at different periods of development. 
Up to the third month of intrauterine life the cord 
occupies the entire length of the spinal canal ; it ex- 
tends downwards to the lowest limit of the canal. 
But from this time on, as growth proceeds, the 
vertebral column lengthens at a more rapid rate than 
the cord. The spinal cord, therefore, has the appear- 
ance of shrinking in an upward divcetan within its 
canal, and at birth its lower end is usually found to 
be opposite the body of the third lumbar vertebra. 
The attitude assumed by the individual affects to 
a small degree the position of the lower end of the 
cord. Thus, when the trunk is bent well forwards, it 
is noticed that the terminal part of the cord rises 
slightly within its bony canal. 
LUTTE HA, 
Cerebellum 
Luiibar swelling 
of the cord 
Fic. 314.—HuMAN FarTus IN THE THIRD 
MONTH OF DEVELOPMENT, WITH THE At the margin of the foramen magnum the 
in caveat Fi CORD EXPOSED syinal cord becomes continuous with the medulla 
oblongata of the brain, whilst below, it tapers. 
rapidly to a point and forms a tapering extremity termed the conus medullaris. 
From the end of the conus medullaris a slender glistening thread is prolonged 
— 5 . 
downwards within the spinal canal, and finally anchors the spinal cord to the back 
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