Acoustic excitations, associated with all the special emotions of the period at which they are implanted in the sensorium, thus perpetuate themselves in the form of memories and as a persistent echo of the past. They are thus capable of reviving, with the qualities with which they were previously gifted. Every one knows, indeed, that a musical phrase is sufficient to recall the circumstances in which we heard it for the first time ; that that instantaneous recollection of certain airs heard during childhood, which is often so vivid, is capable of awakening in us the memory of the places and circumstances in which they were first heard ; and that national airs, among peoples who have imbibed the national sentiment in a precise formula, become very aear to those who hear them when far from their coun try, and are like a perfume from their distant home.
2. Besides this special category of acoustic impressions which directly address the suisoriunz, there is another contingent destined to play a most important part in the phenomena of cerebral life—that which directly serves for the manifestations of verbal expression.
In •the first phases of the development of the young child, it is indeed acoustic impressions that first awaken his mind, and lead him to reproduce the sounds that strike his ears. They are stored up in his sen sorium as persistent memories, represent the absent objects that have been named verbally in his presence, and when reproduced by a reflex action of his brain, become the natural excitants of the different phonetic expressions by the aid of which he designates the same objects, as well as the different conditicns affecting his sensibility. It is by means of this series of acts that human speech, the natural daughter of auditory ex citations, becomes developed in us, expresses itself out wardly, and manifests through precise and appropriate sounds the emotions of the sentient personality which is in action.
It amplifies and develops little by little, and becomes in course of time a true vital force, capable of acting at a distance like a charged electric machine, and of discharging upon the sensorium of another person, and modifying by its seductive influence his sensibility as well as his intelligence. By virtue of the energy with which it is projected, and the heat with which it is expressed, it is capable of provoking different emotions at a distance from the spot where it was engendered, and of exciting sympathetic and persuasive effluences which induce a tacit acquiescence on the part of who ever perceives it. It thus creates a sort of automatic consonance between the orator and those who hear him, and becomes the bond of union which links us to our fellows. It is always due to it that men speaking the same language have among them common points of contact, by which their sensoria, the sensitive regions of their whole personality, converse, touch each other, and vibrate in unison.
3. The special contingent of acoustic excitations which reverberates in the purely intellectual regions, becomes the origin of a series of appropriate judgments which we form respecting the timbre and intensity of sounds emanating from the different sonorous bodies around us.
Thus we judge of the specific pitch of a given sound, by dint of a phenomenon of the memory, by juxtaposing in our mind the reminiscence of a past sound of the same nature as the sound that now strikes our ear.
We judge of the intensity of a sound-producing agency by the manner in which it impresses our auditory nerves, of which the sensibility is called into play and perhaps the notion of muscular activity—the work ac complished by the tensor muscles of the tympanum —may play a certain part in this operation.
It is, further, by a reflex effect of the mind and the memory that we arrive at a judgment respecting the distance of a sounding body. We know that when a known sound gradually decreases in intensity, it is be cause the sonorous body is receding, and when, on the contrary it gradually increases, it is because the sonor ous body is approaching. These two acquired notions afford materials for our judgment in a given case.* Evolution of Olfactory Impressions.—Olfactory im pressions, collected from the peripheral plexuses of the corresponding nerves, are directly transmitted, as we have already explained, to a special department of the optic thalamus, the anterior centre. We have already insisted upon the comparatively large volume of this sensorial gan'lion in those vertebrates that present a great development of the olfactory nerves ; upon the multiple connections it effects with the grey substance of the septum lucidum and mamillary tubercles ; and, finally, upon the indirect relations which unite it to the regions of the sphenoidal lobe, and in particular to those of the grey substance of the hippocampus.t The olfactory nerves transmit to the sensorium the specific and unanalysable notion of odours. They communicate to it at the same time a special coefficient of pleasantness or unpleasantness, according as the inci dent excitation has gratified or run counter to their natural sensibility. For this special group of nerves the impres sion agreeably felt is expressed by the word perfume; the impression disagreeably felt by the word stink. These are the two extreme terms between which all the shades of their peculiar sensibility are developed. They ai e incapable of penetrating profoundly into the recesses of our inner sensibility, to excite those grand movements of expansion or depression which are epitomised in the sentiments of joy or sorrow. From this point of view they are very inferior to optic and acoustic impressions, which monopolize the power of exciting the vibrations of the sensitive chords of our human nature. They only excite, then, a limited action of the sensorium on their arrival. On the other hand, if their diffusive power does not extend to the emotional sphere, it is reverberated in a very direct manner throughout both the vegetative sphere and that of the natural sensibility of certain points of the sensorium, and, when examined from this point of view, olfactory impressions have reflex effects which are quite unexpected.