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552 THE NERVOUS SYSTEM. 
which in the cortex is the corresponding part to the retina, should be of greater extent than the 
auditory field, which represents the cochlea, and the olfactory area, which represents a small 
amount of olfactory mucous membrane in the nasal chamber. 
The four sensorial areas, taken together, only form about one-third of the entire cerebral 
surface. The remaining two-thirds of the cortex constitute what Flechsig has termed the 
association centres. The great extent of these in man must be regarded as a special human 
characteristic. These centres differ from the sensorial areas in being exceedingly poorly pro- 
vided with projection fibres. They have little direct connexion with the centres which lie at 
a lower level. Indeed, the only direct bond of union over a very large extent of these association 
areas with lower centres consists of the thalamo-cortical fibres, which pass to them from the 
thalamus. But, on the other hand, they are rich in association fibres, and are linked in the most 
complete and perfect manner by these fibres to the sensorial areas. 
Flechsig regards these association areas as constituting the portions of cortex in which the 
higher intellectual activities are carried on, and he further believes that they exercise an 
important controlling influence over the sense areas. More particularly is this control exhibited 
in the case of the great someesthetic area within which the influence of all bodily impressions is 
received and transformed into consciousness, and within which the impulses which are thereby 
excited take definite form. These impulses, according to Flechsig, are, in a measure, in all 
properly-balanced minds, held in subjection by the higher feelings, which assume shape in the 
association centres. 
In his study of the foetal and infantile brain Flechsig has shown that the fibres of the sensory 
paths become medullated in the first instance ; then the corticifugal fibres which go out from the 
sense areas assume their sheaths of myelin; and, further, that it is not until a month after birth, 
and after the projection fibres in connexion with the sense areas are myelinated, that the associa- 
tion areas become linked on by medullated association fibres with the sense areas. 
DEVELOPMENT OF THE PARTS DERIVED FROM THE FORE-BRAIN. 
It has been previously noted that the fore-brain very early shows an obscure sub- 
division into a front portion, termed the telencephalon, and a hinder part, called the 
diencephalon, which corresponds more nearly to the original cavity of the fore-brain. 
The cavity of third ventricle is derived from both, and stretches forwards, therefore, to. 
the lamina terminalis, which in its lower part is represented in the adult by the lamina 
cinerea. 
The lateral wall of both sections of the primitive fore-brain shows very distinctly the 
subdivision into a dorsal or alar and a ventral or basal lamina. The groove which 
indicates this separation is the sulcus of Monro, and is evident even in the adult brain. 
Alar Lamina.—The alar part of the lateral wall of the telencephalon is pushed out 
to form the diverticulum, which ultimately constitutes the cerebral hemisphere, and thus 
from a very early period the primitive position of this part of the lateral wall is indicated 
by the wide foramen of Monro, or aperture of communication between the cavity of the 
cerebral hemisphere and the third ventricle. 
The alar part of the lateral wall of the diencephalon is utilised for the development of 
the thalamus, the epithalamus, and the metathalamus. Of these the optic thalamus is 
derived from the anterior and by far the greatest part of the alar wall. It arises as a 
large oval swelling, which gradually approaches its fellow of the opposite side, and thus 
diminishes the width of the third ventricle. Finally, the two bodies come into contact in 
the mesial plane and cohere over an area corresponding to the gray commissure. This 
occurs about the end of the second month. 
From that section of the lateral wall to which the name of metathalamus is given the 
two geniculate bodies arise. Each of these shows, in the first place, as a depression on 
the inside, and a slight elevation on the outside, of the wall of the diencephalon. As the 
thalamus grows backwards, it encroaches greatly upon the territory occupied by the geni- 
culate bodies. It thus comes about that in the adult brain the internal geniculate body 
seems to hold a position on the lateral aspect of the mesencephalon, whilst the external 
geniculate body, viewed from the surface, appears to be a part of the thalamus. 
From the epithalamic region of the wall of the diencephalon are developed the pineal 
gland, its peduncle, and the habenular region. These parts are relatively much more 
evident in the embryonic than in the adult brain. The pineal body is developed as a 
diverticulum of the posterior part of the roof of the diencephalon. Viewed from the 
dorsal aspect of the brain-tube, this diverticulum shows in the first instance as a rounded 
elevation, from either side of which a broad ridge runs forwards. This ridge becomes 
the tenia thalami, whilst in the region of its junction with the pineal elevation the 
trigonum habenule takes shape. The pineal diverticulum ultimately becomes solid, but 
a small portion of the original cavity is retained as the recessus pinealis of the third 
ventricle. 
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