542 THE NERVOUS SYSTEM. 
The posterior limb of the internal capsule is placed between the optic thalamus 
and the lenticular nucleus, and it extends backwards for a short distance beyond 
the hinder end of the putamen on the outer side of the posterior part of the 
thalamus and of the tail of the caudate nucleus. The posterior limb, therefore, is 
spoken of as consisting of a lenticular and a retrolenticular part. 
The lenticular part of the posterior limb is composed of both corticipetal and cortici- 
fugal fibres. The corticipetal fibres enter the internal capsule from the outer aspect of 
the DAK thalamus, and are composed of thalamic fibres which arise within the thalamus, 
and also, in all probability, certain long tracts (such as portion of the mesial fillet and 
portion of the superior cerebellar peduncle), which are said to be carried up through the 
thalamus into the internal capsule. 
The a fibres form a well-defined tract, which occupies the anterior half of the 
lenticular part of the internal capsule. This is the great motor or pyramidal tract 
descending from the Rolandic area of the cortex. The fibres, which go to the nucleus of 
the facial nerve, lie close to the genu, and behind these are the fibres which go to the 
hypoglossal nucleus ; still further back are pyramidal fibres, which enter the spinal cord 
and end around the motor cells of the anterior horn of gray matter. This pyramidal 
tract has been observed occupying the middle part of the crusta of the crus cerebri, into 
which it passes directly from the internal capsule. It is doubtful if there are any other 
corticifugal fibres in the lenticular part of the posterior limb of the internal, capsule. 
Some observers, however, consider that with the pyramidal fibres there are cortico-pontine 
fibres from the Rolandic area—fibres which end below in the nucleus pontis (van Gehuchten). 
The retrolenticular part of the posterior limb contains: (1) the fibres of the optic 
radiation as they pass to establish their connexions with the thalamus, superior quadri- 
geminal body, and corpus geniculatum externum ; (2) the fibres of the auditory radiation, 
or those which connect the auditory cortical field in the temporal lobe with the corpus 
eeniculatum internum (Fig. 357, p. 480); (3) the temporo-pontine tract, which is com- 
posed of fibres which take origin in the two upper convolutions of the temporal lobe and 
pass through this section of the internal capsule to reach the outer part of the crusta of 
the crus cerebri. Through this they reach the ventral part of the pons, in the gray matter 
of which they end. 
When the fibres of the internal capsule are traced upwards they are found to 
spread out widely from each other in a radiating or fan-shaped manner, so as to 
reach the various convolutions of the cerebral hemisphere. This arrangement 1s 
termed the corona radiata. The callosal system of fibres, as they proceed into the 
hemisphere, also radiate, and they intersect the fibres of the corona radiata (Fig. 
403, p. 540). 
External Capsule.—The thin lamina of white matter between the outer aspect 
of the putamen and the claustrum is called the external capsule. This joins with 
the internal capsule in front of and behind the putamen, and in this manner the 
lenticular nucleus is encapsulated by white matter. 
INTIMATE STRUCTURE OF THE CEREBRAL HEMISPHERE. 
The cerebral hemisphere is composed of an external coating of gray matter, 
termed the cortex, spread over an internal mass of white matter, which is called the 
medullary centre. ‘I'he cortex is of peculiar interest, seeing that there is good reason 
for believing that in it the higher functions of the brain, or those which may be 
classed under the general designation of the intellectual functions, take place. It 
is within the same layer of gray matter that the influence of those external i impres- 
sions, which gain access to the cerebro-spinal axis through the senses, finally take 
shape as consciousness; and in it also are placed the centres which carry on the 
psycho-motor functions. The white medullary centre is composed of nerve-fibres 
which constitute the paths along which the influence of impressions js carried to 
and from the cortex, and from one part of the cortex to another. 
THE CEREBRAL CORTEX. 
The gray cortex is spread over the entire surface of the cerebral hemisphere, but 
it does not form a layer of equal thickness in all localities. At the summit of a 
OS ee 
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