Let us consider the mode in which the excretion of semen takes place in the normal state. It is the remote consequence of a voluntary action, and the immediate result of involuntary contractions. The venereal sense is excited voluntarily, either by copulation, or by applying the hand: this excitement is carried to as great an extent as possible; and then a crisis, entirely independent of the will, terminates it. This crisis occurs sooner or later. It may even be quickened or retarded by the will, which may excite or modify the venereal sense; but when it does take place, it is always by involuntary contractions--that is, by a true convulsion. This last action has two well marked periods. In the first, the semen passes from the seminal vesicles into the urethra; in the second, this liquid is violently expelled. The contraction of the seminal vesicles--and perhaps, also, that of the levatores ani muscles--are the powers by which the semen comes into the urethra. The ejaculation is caused by the muscles of the perinœum, and particularly by the bulbo-cavernosus muscle. The swelling and hardness of the corpus cavernosum furnish this muscle with a point of resistance, which enables it to compress more efficiently the semen with which the urethra is filled; and the straightening of this canal, by the erection, renders the expulsion of this fluid more easy. All these motions take place by jerks; and, we repeat it, convulsively, without the aid of the will. The involuntary excretion of the semen, the morbid pollution, may take place sometimes in the manner described, sometimes in another mode. In the first case, it differs from what occurs in the normal state, only not being preceded by those acts which are performed voluntarily by man. Secondly, the semen is excreted without any convulsive effort; it flows like the tears, the saliva, the bile. The semen comes into the urethra, and escapes from it merely because it is there. There is no ejaculation: and this is easily conceived of; for the genital organs are not sufficiently excited, to cause the ejaculatory powers to be convulsed, as is proved by the excessive weakness of the venereal sensation. And, secondly, one of the indispensable conditions of the ejaculation--erection of the penis--does not exist. There are, then, two kinds of involuntary pollutions; one which is _convulsive_, and the other which is _not so_. Between these two kinds, there are intermediate degrees, in which spermatorrhœa partakes more or less of one or the other. These degrees often mark the passage from convulsive spermatorrhœa to that which is not convulsive; for the latter has generally been preceded by the former. We shall see hereafter, that the existence of one of these affections does not forbid that of the other; and that the ejaculation of semen is possible in some individuals who present habitually an insensible flow of this fluid.