438 THE NERVOUS SYSTEM. 
between the tract of Burdach and the postero-median septum. Later on the lateral and 
anterior columns are increased in bulk by the descent into them of the pyramidal tracts 
from the brain. 
The gray matter, in the first instance, is chiefly massed in the basal lamina; but as 
the posterior columns of white matter begin to take shape it extends backward, and in 
the course of time the posterior horns are developed. 
The manner in which the dendritic processes of the neuroblasts are developed has 
been sufficiently described (p. 417). The ensheathment, also, of the axons by medulla 
has been referred to, and the fact that the different tracts of fibres receive their medul- 
lary sheaths at different periods mentioned. It is now only necessary to state that the 
order of myelinisation of the several tracts is as follows :—(1) Fibres of the anterior nerve- 
roots ; (2) tract of Burdach ; (3) fibres in the basis-bundles ; (4) tract of Goll; (5) direct 
cerebellar tract ; (6) tract of Gowers; (7) pyramidal tracts (Kahler). 
Development of the Median Fissures and of the Central Canal.—As the 
anterior horns of gray matter covered by the anterior columns of white matter increase in 
size, the anterior surface of the cord on either side of the mesial plane bulges forwards, and 
the antero-median fissure is produced as the natural result. 
The manner in which the postero-median fissure comes into existence is not fully 
understood, but the majority of embryologists believe that it is produced by the approxi- 
mation and fusion of the walls of the posterior part of the primitive cavity of the neural 
tube. The postero-median septum would thus appear to be formed of spongioblastic tissue. 
If the above view of the formation of the postero-median fissure be correct, it must be 
evident that the central canal of the cord does not represent the whole of the primitive 
cavity of the early neural tube, but only the anterior portion of it. 
Among those observers who do not hold that the central canal and posterior fissure have this 
mode of origin the most prominent is Professor A. W. Robinson, of King’s College ; and he has 
brought forward evidence which seems to indicate that it is doubtful if the fusion of the walls 
of the posterior part of the canal, referred to above, takes place. Certainly the arrangement of 
the ependymal elements of the postero-median septum, as seen in the preparations of Cajal 
and v. Lenhossek, are extremely difficult to understand on the fusion theory. They run in the 
antero-posterior direction, whereas, if fusion has taken place, most of them would present a 
transverse arrangement, and thus lie at right angles to the postero-median septum. 
THE BRAIN OR ENCEPHALON. 
The brain is the enlarged and greatly modified upper part of the cerebrospinal 
nervous axis. It is surrounded by the same membranes that envelop the spinal 
cord (viz. the dura mater, the arachnoid mater, and the pia mater), and it almost 
completely fills up the cavity of the cranium. So closely, indeed, is the skull 
capsule moulded upon the brain that the impress of the latter is almost everywhere 
evident upon the deep surface of the cranial wall. The relations, therefore, of 
cranium to brain are totally different from those presented by the vertebral canal 
to the spinal cord. As we have noted, the cord occupies only a part of its bony 
case; and there is not only a wide and roomy space between the arachnoid mater 
and pia mater, but also an interval of some width between the dura mater and the 
walls of the vertebral canal. 
General Appearance of the Brain.—When viewed from above the brain 
presents an ovoid figure, the broad end of which is directed backwards. — Its 
greatest transverse diameter is usually found in the neighbourhood of that part 
which lies between the two parietal eminences of the cranium. The only parts 
which are visible when the brain is inspected from this point of view are the two 
convoluted cerebral hemispheres. These present an extensive convex surface, which 
is closely applied to the deep aspect of the cranial vault, and are separated from 
each other by a deep median cleft, termed the great longitudinal fissure, which 
extends from the front to the back of the brain. 
The inferior aspect of the brain is usually termed the “base.” It presents an | 
uneven and irregular surface, which is more. or less accurately adapted to the 
inequalities on the floor of the cranium. Upon this aspect of the brain some of 
its main subdivisions may be recognised. Thus behind is seen the short cylindrical 
portion, called the bulb or medulla oblongata, through which, at the foramen magnum, 
