Much more important than the general motor phenomena, more purposive though involuntary, are the specifically sexual muscular movements. From the very beginning of detumescence, indeed, muscular activity makes itself felt, and the peripheral muscles of sex act, according to Kobelt's expression, as a peripheral sexual heart. In the male these movements are fairly obvious and fairly simple. It is required that the semen should be expressed from the vesiculæ seminales, propelled along the urethra, in combination with the prostatic fluid which is equally essential, and finally ejected with a certain amount of force from the urethral orifice. Under the influence of the stimulation furnished by the contact and friction of the vagina, this process is effectively carried out, mainly by the rhythmic contractions of the bulbo-cavernosus muscle, and the semen is emitted in a jet which may be ejaculated to a distance varying from a few centimeters to a meter or more.
With regard to the details of the psychic sides of this process a correspondent, a psychologist, writes as follows:—
"I have never noticed in my reading any attempt to analyze the sensations which accompany the orgasm, and, as I have made a good many attempts to make such an analysis myself, I will append the results on the chance that they may be of some value. I have checked my results so far as possible by comparing them with the experience of such of my friends as had coitus frequently and were willing to tell me as much as they could of the psychology of the process.
"The first fact that I hit upon was the importance of pressure. As one of my informants picturesquely phrases it—'the tighter the fit the greater the pleasure.' This agrees, too, with their unanimous testimony that the pleasurable sensations were much greater when the orgasm occurred simultaneously in the man and woman. Their analysis seldom went further than this, but a few remarked that the distinctive sensations accompanying the orgasm seem to begin near the root of the penis or in the testes, and that they are qualitatively different from the tickling sensations which precede them.
"These tickling sensations are caused, I think, by the friction of the glands against the vaginal walls, and are supplemented by other sensations from the urethra, whose nerves are stimulated by pressure of the vaginal walls and sphincter. The specific sensation of the orgasm begins, I believe, with a strong contraction of the muscles of the urethral walls along the entire length of the canal, and is felt as a peculiar ache starting from the base of the penis and quickly becoming diffused through the whole organ. This sensation reaches its climax with the expulsion of the semen into the urethra and the consequent feeling of distention, which is instantly followed by the rhythmic peristaltic contractions of the urethral muscles which mark the climax of the orgasm.
"The most careful introspection possible under the circumstances seems to show that these sensations arise almost wholly from the urethra and in a far less degree from the corona. During periods of great sexual excitement the nerves of the urethra and corona seem to possess a peculiar sensitivity and are powerfully stimulated by the violent peristaltic contractions of the muscles in the urethral walls during ejaculation. It seems possible that the intensity and volume of sensation felt at the glans may be due in part to the greater area of sensitive surface presented in the fossa as well as to the sensitivity of the corona, and in part to the fact that during the orgasm the glans is more highly congested than at any other time, and the nerve endings thus subjected to additional pressure.
"If the foregoing statements are true, it is easy to see why the pleasure of the man is much increased when the orgasm occurs at the same time in his partner and himself, for the contractions of the vagina upon the penis would increase the stimulation of all the nerve endings in that organ for which a mechanical stimulus is adequate, and the prominence of the corpus spongiosum and corona would ensure them the greatest stimulation. It seems not improbable that the specific sensation of orgasm rises from the stimulation of the peculiar form of nerve end-bulbs which Krause found in the corpus spongiosum and in the glans.
"The characteristic massiveness of the experience is probably due largely to the great number of sensations of strain and pressure caused by the powerful reflex contraction of so many of the voluntary muscles.
"Of course, the foregoing analysis is purely tentative, and I offer it only on the chance that it may suggest some line of inquiry which may lead to results of value to the student of sexual psychology."
In man the whole process of detumescence, when it has once really begun, only occupies a few moments. It is so likewise in many animals; in the genera Bos, Ovis, etc., it is very short, almost instantaneous, and rather short also in the Equidæ (in a vigorous stallion, according to Colin, ten to twelve seconds). As Disselhorst has pointed out, this is dependent on the fact that these animals, like man, possess a vas deferens which broadens into an ampulla serving as a receptacle which holds the semen ready for instant emission when required. On the other hand, in the dog, cat, boar, and the Canidæ, Felidæ, and Suidæ generally, there is no receptacle of this kind, and coitus is slow, since a longer time is required for the peristaltic action of the vas to bring the semen to the urogenital sinus. (R. Disselhorst, Die Accessorischen Geschlechtsdrusen der Wirbelthiere, 1897, p. 212.)