A. In Nonrational Animals

427. - By the name sexual instinct is meant an innate propensity toward sexual acts. This disposition is generally habitual or latent; it passes into act with the aid of stimuli which can be external or internal.

What these are has already been stated in the body of the book, and we shall speak of them again below.

But you should observe that in animals the proximate power for the venereal act and the connected excitation is not present at every time of the year.

But when internal secretion makes them ardent or in heat, then, if an external stimulus is present, for example the odor of the body of the other sex, they are impelled toward manipulations which induce orgasm, by which they at the same time resolve the preceding affective state, which is one of a certain tension or turgescence. And even apart from external occasions, instinct, through an innate association, excites a phantasm of the manipulations which are to be performed about the sexual organs of the other sex.

Therefore, a sufficiently accepted theory distinguishes three things of which sexual instinct is composed: turgescence, manipulation, detumescence.

Moreover, in nonrational animals sexual instinct is determined only toward venereal acts, or toward perceiving their own pleasure from copulation. And it is irresistible, insofar as the animal cannot resist it by its own effort: external fears alone can overcome the impulse.