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DEVELOPMENT OF THE SPINAL CORD. 435 
From this crossing of the pyramidal tracts, it results that the destruction of the fibres which 
compose them as they descend in one side of the brain must result in paralysis of the muscles 
supplied by the efferent nerves of the opposite side of the cord. 
It is well to note that the fibres of both pyramidal tracts are not medullated until the time of 
birth. They are the latest of all the cord-tracts to myelinate. 
The anterior basis-bundle (fasciculus anterior proprius), like the lateral basis- 
bundle, is composed largely of fibres which arise from the cells of the gray matter 
of the cord, and act the part of intersegmental association fibres. 
Summary of the Constitution of the White Matter of the Cord.—The 
white columns of the cord are formed of two kinds of nerve-fibres :— 
1. Those whicli enter the cord from without. 
2. Those which take their origin from the cells within the gray matter of the 
cord itself. 
Under the first category we include (a) the greater part of the fibres of the 
posterior column (columns of Burdach and Goll), which arise from the cells of the 
spinal ganglia, and which enter the cord as the posterior nerve-roots; and (0) the 
crossed and direct pyramidal tracts which come from the motor cells of the cerebral 
cortex. 
The fibres which arise within the gray matter of the cord may be classified 
thus: (@) Fibres which pass out from the cord as efferent nerves (anterior nerve- 
roots); (0) fibres which form long tracts and pass up the cord to enter the brain 
(direct cerebellar tract and the tract of Gowers); (¢) fibres which form short tracts, 
linking together different segments of the cord (intersegmental association fibres in 
each of the three columns of the cord). 
Anterior White Commissure.—The anterior commissure is composed of 
medullated nerve-fibres passing from one side of the cord to the other and entering 
the anterior horn of gray matter, and also the anterior column. It is to be regarded 
more as a decussation than as a commissure, and its width, which varies somewhat 
in different regions, fluctuates in correspondence with the diameter of the cord. 
Amongst the fibres which cross in the anterior commissure may be mentioned: (1) The fibres 
of the direct pyramidal tract; (2) collaterals from both the ventral and lateral columns ; (3) axons 
of many of the cells of the , aray matter ; (4) the dendritic processes of some of the mesial ventral 
cells. 
Posterior Gray Commissure.—Although this is composed of gray matter with 
a large admixture of neuroglia, numerous transverse nerve-fibres pass through it, so 
as to bind the cells of one side of the cord to those of the other. 
DEVELOPMENT OF THE SPINAL Corp. 
In the chapter upon General Embryology it has been pointed out (p. 19) that the 
brain and cord first take shape in the form of a tube of ectoderm, which receives the name 
of the neural tube. Three expansions, placed one behind the other at the cephalic end of 
the tube, represent the early brain ; whilst behind these primitive cerebral vesicles comes 
the elongated narrower part of the tube, which at this stage represents the spinal cord. 
By a developmental process, which we now have to study, the walls of this portion of the 
neural canal give rise to the various elements which build up the substance of the cord, 
whilst a portion at least of the primitive cavity is preserved as the central canal of the 
cord. The account which is here given of the development of the cord is taken almost 
entirely from the writings of Professor His of Leipzig. 
When first formed, the neural tube is compressed from side to side and presents an oval 
outline in transverse section (Fig. 14, p. 20). The two lateral walls are very thick, whilst 
the narrow dorsal and ventral portions of the wall are thin, and are termed the mid-dorsal 
and mid-ventral lamine. ‘The cavity of the tube in transverse section appears as a 
narrow slit. At this stage the wall of the neural tube is formed of a series of elongated 
neuro-epithelial columnar cells, closely applied to each other and extending throughout the 
whole thickness of the wall. The inner ends of these long columnar cells unite to form a 
delicate membrane termed the internal limiting membrane, which lines the lumen of the 
tube, whilst their outer ends present a similar relation to an external limiting membrane, 
which invests the outer surface of the tube. The name of spongioblasts is given to these 
cells, and they soon develop in such a manner as to form the sustentacular framework of 
the growing cord. Between their inner parts, immediately subjacent to the internal 
A, 
