Under the congenital malformations of the brain we have already described several conditions which might be included in the designation congenital atrophy or aplasia. We have also seen that in Dementia paralytica there is an atrophy of the brain as a whole.
Senile atrophy has somewhat similar characters to that last mentioned. Occasionally the brains of old people undergo a general shrinking, and the space is made up partly by dropsy of the ventricles and oedema of the membranes and partly by thickening of the cranial bones. The atrophy is sometimes partial.
It is not necessary to give here a detailed account of these, as they have mostly been incidentally considered.
The White substance of the brain undergoes a process of atrophy under various circumstances - in softening of the brain, in sclerosis, etc. It also presents a condition which Lockhart Clarke has designated Granular disintegration. This occurs in the neighbourhood of bloodvessels, and is probably due to exudation from them. The white substance degenerates into an indefinite granular material.
The Ganglion cells are frequently the seat of atrophy and degeneration. They may undergo a Simple atrophy, shrinking and losing their processes. But very commonly the atrophy is accompanied by pigmentation, and so a Pigmentary degeneration is the result. Virchow was the first to describe a Calcification of the ganglion cells. This condition seems to be of frequent occurrence when these cells are suddenly deprived of vitality. It was found by Virchow originally in cases of commotio cerebri, but has since been seen in softening of the brain, in acute poliomyelitis anterior, etc. The ganglion cells, having died, and having undergone coagulation-necrosis, become infiltrated with lime, like other dead structures. A Hyaline or Colloid degeneration of the ganglion cells has also been described, especially in cases of insanity.
Hyaline changes in the vessels of the brain, more especially the arteries and capillaries, are frequent in the insane, more particularly in chronic cases.
These are similar to those already described as occurring in the spinal cord, and they concern chiefly the pyramidal tract. This tract degenerates when it is cut off from the motor convolutions at whatever level. This severance is the result of destructive lesions, usually haemorrhage or softening, the most frequent seat of such lesions being, as already mentioned, the region of the corpus striatum. In cases of extensive destruction of the motor convolutions the secondary degeneration has been traced downwards through the pyramidal tract to end only in the spinal cord. A tract of grey degeneration is found in the parts of the corona radiata corresponding with the lesion. It extends to the internal capsule, occupying the anterior two-thirds of its hinder limb (ik, Fig. 321), thence it passes to the crus cerebri, occupying the middle two-fifths of the crusta, extending from the surface below nearly to the substantia nigra above. In the pons the tract is anterior, and is covered over by transverse fibres, which also pass in and separate the bundles. The course in the medulla oblongata and cord has already been traced.
I n some cases of unilateral cerebral disease there is a degeneration on both sides of the cord, the motor centres of each cerebral hemisphere apparently representing both sides of the body. The degeneration, however, is much more marked in the pyramidal tract on the side opposite to the lesion.
Secondary degeneration also occurs in the fibres passing from the parts anterior to the motor area, namely, those in front of the ascending frontal convolution. These fibres pass through the anterior limb of the internal capsule and the inner part of the crusta of the peduncles, to-end in the grey matter of the pons where their degeneration ceases. These fibres are probably prolonged to the cerebellum, forming communications between the frontal convolutions and the cerebellum (fronto-cerebellar fibres).
If of long standing there may be considerable shrinking in the parts-affected by the secondary degeneration.