GENESIS AND EVOLUTION OF MEMORY.
IN order that the processes of cerebral activity which constitute memory shall be evolved according to their natural laws, it is necessary that the peripheral regions of the system which collect and transport sen sorial impressions, on the one hand, and the central regions which transform and absorb them, on th% other hand, shall be reciprocally in suitable conditions of physiological conductility and receptivity.
i. It is indeed in the peripheral regions, in the midst of the ultimate nervous expansions, that the activity of the central regions finds its regular Thence it is that all the stimulations destined to set them in motion proceed.
When an external excitation is reverberated to any point whatever of their essential structure—whether it be a sonorous wave thrilling through the acoustic expansions, or a luminous wave becoming extinguished in the regions of the retina, or any direct stimulus which sets in vibration the sensitive nerves of the skin and mucous membranes —immediately this purely physical excitation is trans formed on the spot by the peculiar action of the nervous plexus in erethism. It absorbs it, transforms it into nervous vibrations, and to some extent animalizes it by incorporating it with the organism.
Now, since the peripheral nerve-cells, as we have said, retain in themselves, like phosphorescent gleams, the record of those stimulations which have first set them vibrating, the result is that these persistent impressions become, without our knowledge, like a store of latent peripheral reminiscences, which hold the partner cells of the central regions in a sort of persistent vibratory sympathy. They in their turn assist the action of the central memory, and thus become a means of physio logical reinforcement designed to vivify and maintain its activity.
This solidarity between the peripheral and central regions of the system is so real, that when the former fail, the functionment of the central regions is at the same time interrupted.
When the sensitive peripheral regions are in a state of central perception ceases. There is no persistent reminiscence in the sensorium, because the trace of the persistent peripheral impression has not been registered. Touch, pinch, excite the skin of a hysterical patient in any way you please, if the eyes be closed, she will retain no remembrance whatever of the cutaneous excitations, because her peripheral nervous plexuses being stupefied, will not have been able to transmit to the sensoriunz anything that has taken place in their internal structure. I have often seen general paralytics,
attacked with transient anaesthesia of the gustatory and pharyngeal nerves, bitterly complain to me that they had not been given a particular dish at their meal, I having been present when they had partaken of the food which they declared they had not received. Then again, the absence of sensibility in the peripheral region causes the sensorial impression not to be absorbed on the spot, nor directly transmitted to the central regions by its habitual channels.
In order that the sensorial impression shall produce the desired effects in the plexuses of the sensorium, and shall be clearly perceived, it is necessary then that the peripheral plexuses, which are its true gates of admission into the organism, shall be in a condition of receptivity and peculiar erethism, that their natural sensibility shall be directly awakened, and that there shall be on their part an active and prolonged participation when the stimulation from without arrives.
Every one knows, indeed, that a slight and fugitive impression leaves but insignificant traces of its passage ; that an incessant repetition of the same impressions is necessary, in order that they shall be retained in a stable manner ; and that it is only by dint of forgetting, that we come to have certain details present in our minds which escape us and which it has been necessary to learn again and again. The repetition of the same peripheral impressions, the repeated view of the same objects. the hearing of the same sounds, become there fore indispensable fundamental conditions of the pre servation of reminiscences ; and from this point of view the reminiscences emanating from the sensorial plexuses, the memory of the senses, as they are pedantically called, are the most energetic stimulations of mental memory.* On the other hand, in order that the impression per sistent in the peripheral plexuses shall produce a durable impression in the central regions, the preceding condi tions of centrifugal impression are not the only ones necessary. It is necessary that there shall be some thing more on the part of these same central plexuses of the effective participation or intimate association of their sensibility with the peripheral exci tations which thus throw it into agitation.