CHAPTER XIII

THE LOCALIZATION OF MEMORY

I HAVE endeavoured to show, in the previous chapters, that every impression received is registered in a certain position in the brain; that all impressions, received at the same time, form component parts of one impression, or contiguous impressions; that impressions received about the same time are registered in contiguity; how a motor memory is formed; how the faculties, by producing varying intensities of sensory impressions, develope special memories; how the sensory and motor memories become associated, indirectly and directly; how association and revival of impressions takes place; how it is that the old forget recent events, whilst remembering those of their youth, etc.

Therefore, from a consideration of the previous chapters, it will be seen that the required centres of memory must occupy a position below, and in intimate. connection with, the seat of the perceptive, reasoning, and imaginative faculties. It will also be seen that they must have an intimate relation with the nerves of special and common sensation. On account of the varying states of perception, an independent arterial blood-supply will be required, and a capacity for

expansion separate from that of the remainder of the brain.

Are there any parts of the brain in a position anatomically and physiologically to fulfil the above requirements? There are, namely, the optic thalami and the corpora striata, as the seats of sensory and motor memory, respectively.

Anatomical Relations of the Optic Thalami and

Corpora Striata. (See Frontispiece.)

The optic thalami are two oval-shaped masses of gray matter, situated just above and intimately connected with the crura cerebri and nerves of special and common sensation, which terminate, if not entirely, in a great part, in these bodies. Above, they are intimately connected with the cerebral hemispheres, and horizontally with the corpora striata. The optic thalami of opposite sides are connected by the middle and posterior commissures.

The corpora striata each consists of two nuclei of gray matter, the nucleus caudatus and the nucleus lenticularis, with an intervening band of white substance, the internal capsule, which also separates the nucleus lenticularis from the optic thalamus. They are situated just above the crura cerebri, with the motor tract of which they are intimately connected, and with the cerebral hemisphere and optic thalamus of their own side.

The nuclei caudati, and especially the optic thalami, are in such a situation as to be able to expand or contract without interference from, or interfering with, the remaining portions of the encephalon. Despite their intimate relations to the surrounding parts, they occupy a position, nearly exposed in the third and lateral ven

tricles, and at the base of the brain: so any expansion of these bodies simply displaces the subarachnoid fluid in the spaces at the base of the brain and in the lateral ventricles.

An independent blood supply is necessary for an independent expansion and contraction, and this, these bodies possess. They are supplied by long thin arteries, which pass upwards through the perforated spaces, from the circle of Willis, without anastomosing either amongst themselves or with the cortical vessels, in any way. The optic thalami are supplied with arterial blood by branches from the posterior cerebral and anterior and posterior communicating arteries. Both the nuclei lenticulares and the nuclei caudati, of the corpora striata, are supplied by arteries from the middle cerebral, and the nuclei caudati receive additional branches from the anterior cerebral.

The veins from these bodies do not accompany the arteries, but chiefly terminate in the venæ galeni, which empty into the straight sinus.

I have omitted any further anatomical details regarding these bodies, as they are irrelevant to the subject of memory, and so, for more detailed information, I must refer the reader to the ordinary text-books on anatomy.

With regard to experiments on animals, those on the optic thalami have yielded purely negative results, as far as the muscles are concerned, whilst those on the corpora striata have produced general contractions of the muscles on the opposite side of the body.

Minute Anatomy of the Basal Ganglia.

The optic thalami consist of a central mass of gray matter containing numerous multipolar ganglion cells,

and a superficial white cortical portion. Large tracts of medullated nerve-fibres are found to terminate in the optic thalami, both from the cerebrum above, and the nerves below.

The nuclei of the corpora striata have a similar minute anatomy, consisting of gray matter containing groups of multipolar ganglion cells. Numerous tracts of medullated nerve-fibres are found to arise or terminate in this gray matter, from the motor nerves below, and the cerebral hemispheres above.

Here we have two bodies, conforming in every anatomical particular to the necessary requirements, and having that minute structure, consisting of masses of ganglionic gray matter, which, no matter in what part of the nervous system it is found, always has some definite function, other than that of conduction.

Comparative Anatomy.

We find that the size of the basal ganglia bears no necessary relation to the cerebrum, but is proportionately larger in the lower animals, thus corresponding to the evidence of comparative psychology, which shows that the memory in them is in the same proportion greater than the cerebral functions.

With regard to disease of the optic thalamus of one side, the only special symptom appears to be impairment of impressions received from the special senses on the opposite side of the body; and when the optic thalamus alone was affected, pain, as a symptom, was invariably absent.

Callender,' in an article containing the details of

1 St. Bartholomew's Hospital Reports, Vol. V.

fifty cases of disease affecting the basal ganglia, finds that hæmorrhage into the optic thalami, or corpora striata, is usually months in proving fatal, and that the patient very rarely complained of pain.

In every instance in which pain was a symptom, the gray substance of the cerebral cortex was affected as well.

We should theoretically expect that the above would be the effect of disease of one optic thalamus. As all impressions are received in both optic thalami, we should not expect to find any very great loss of memory; what did occur was probably overlooked. Symmetrical disease of the brain is rare, and in the above cases in not a single instance were both optic thalami affected. As the optic thalami are the centres for the reception of impressions, we should naturally expect to find that the special senses on the opposite side were impaired.

The above is strongly against the theory that the optic thalami are the seats of common sensation, no pain being found when they alone were diseased. No pain would be expected from disease of the seat of memory.

As there is a special provision for the enlargement or diminution of the optic thalami, the excess of fluid passing from the lateral and third ventricles along the aqueduct of Sylvius to the fourth ventricle, and then passing into the general subarachnoid space through the foramen of Majendie, we should expect definite symptoms from obstruction in any part of this track.

The optic thalami will contain the largest amount of blood, and, therefore, occupy the greatest cubical space when a large number of impressions are being revived