For this special order of excitations the primordial period of erethism which incarnates them in the organism, instead of being a local and instantaneous phenomenon like those of general sensibility, or vision, for instance, is divided into successive moments. It is effected by means of special erectile apparatuses, which develop, and complete it, and insensibly lead up to a condition of supreme exaltation. Once the external impression is incarnated in the sensitive plexuses, once the notion of physical pleasure is developed with all its conse'uences, it ceases to be itself, through dynamic exhaustion, sheer fatigue of the nerves, as we have seen the retina when fatigued becomes insensible to the contemplation of certain luminous rays.
The process of physical pleasure undergoes, then, a series of phases through which it only gradually arrives at its complete expansion.
It begins locally, in the peripheral plexuses, with a period of extreme erethism, from the intimate connection of the sexes, through the mysterious conjunctions of the apparatuses of organic life ; it is at the same time enriched by the action and sympathetic participation of all the diffuse sensibilities of the organism which are thrown into agitation, those of the tactile surfaces, the hands, the lips, which all combine to enhance its pri mitive energy ; it advances towards the central regions, as a true synthesis of all the impressionable ele ments of our nature in vibration, is propagated through the whole length of the spinal axis by means of the conducting fibres which convey it, and, after passing through its final stages in the intermediate grey regions of the optic thalamus, it is dispersed in the different zones of the sensoriurn, carrying with it the shock of joy and satisfaction which intrinsically charac terizes it.
Like all the other sensorial impressions, the excita tions of physical pleasure affect both the sphere of psychical activity and that of intellectual activity proper.
i. The excitations of physical pleasure, which, as regards the living being, represent the fundamental elements of the prime function which has for its end the reproduction of the species, arrive in the sensorium accompanied by an enormous contingent of sensations emanating from different regions simultaneously in a condition of erethism. They essentially carry with them impressions of joy and happiness, and produce like conditions in the elements of the sensorium; becoming during the period of puberty a dominant note which vibrates above all the rest, which gives its tone to all our actions, all our discourses ; and which, when it happens to be set vibrating with special intensity, extinguishes all the rest by its intensity and splendour.
Psychic, ideal love, and physical love are, then, the ultimate links of one and the same chain of which the elements are uninterruptedly connected. It is a regular physiological process, which has its roots in the intimate connection of the sexes, and its expansion in the most elevated regions of psycho-intellectual activity. In evolving itself throughout the organism, it thus involves the incidental calling into play of all the apparatuses of the essential life of the living creature, and their harmonious co-operation.
It has, then, its raison d'etre in a purely physical plea surable excitation, which presides over its origin and marks its first stage. It is a fleeting and transient desire, which is born, passes, and fades away as soon as the physical demands for pleasure which gave it birth are appeased ; but as the same physical needs arise again, through the necessary laws of the movement of life in living beings, the same voluptuous desires simul taneously arising in the sensoriunz, it follows that the reiteration of the same physical satisfactions finally leaves upon the sensorium itself a persistent and con tinuous impression, vibrating like an echo of the past, and thus maintaining a durable and uninterrupted sentiment. Thus it is that love, a sentiment transient and ephemeral as the pleasure which gave it birth, fixes itself perma nently, and lives with a life of its own. The reiteration of the satisfaction of physical pleasure, obtained from the same sources as formerly, and new desires resuscitate and reinvigorate it, and become the elements of its continuity and its persistence.
Conjugal love, thus made an abiding sentiment in the sensorium, becomes in its turn the physiological pivot around which a new generation of consecutive senti ments gravitates.
Thus, by the natural fact of the evolution of the living organism, physical love, which was at first all concentrated upon a single head, upon the being which gave it birth—its end being the propagation of the species—when once this end is attained is insensibly extended to the offspring, which is the flesh of the flesh of this being, and the veritable proliferation of her substance. The sentiments of the family which are then developed, lead man's emotional nature into the inevitable cycle of the affection of parents for their children, that inevitable cycle in which we have been preceded by all the generations of our ancestors, and in which all the representatives of the human race are destined perpetually to move.