6. If a person is in good health, and of a proper age, the seminal fluid is rapidly formed. This takes place in those that have not done growing and in those that are grown. Women rarely know when they are first pregnant; for they do not think that they have conceived unless they perceive that the semen has been emitted, suspecting that it ought to be emitted at the same time both by the female and the male; and it escapes their notice, more especially when they think that they are unable to conceive, unless they have become dry, and that which they have received has disappeared entirely; but it sometimes happens that both the male and the female emit more than could possibly disappear, and more than enough for conception. When sufficient has been drawn in and much left out, they become pregnant without knowing it. 7. That it is possible that this should take place, and that the affection does not arise from the whole of the seminal fluid, we may learn from those animals which produce many young ones from a single act of intercourse, or from the case of twins produced by a single act. It is evident that they are not produced from the whole semen, but each place receives some portion of it, but the larger portion is left behind; and if many young are produced from a single act of intercourse, which appears to be the case with swine and with twins, it is evident that the semen cannot come from every part of the body, but it is divided out to each form. It is possible, therefore, that it may be separated from every part of the body, and that the whole may be divided among many, so that it is not possible that all should have every part. The female also projects her semen into the os uteri, where the man also emits his, when he approaches her. From thence she imbibes with inhalation as if it were with the mouth or nostrils; for whatever is not joined to the members is either hollow above and united by a symphysis, or is sucked in from this place by the act of inhalation. For which reason they take care that it should be dry, as if this had happened before. 8. The path along which it passes is thus formed in women. There is a tube enclosed in the body like the penis of the male. The inhalation takes place through this by a small passage above the passage for the urine. When, therefore, they desire sexual intercourse, this part is not in the same condition as it was before. A falling down takes place from this passage, and the fore part of the uterus becomes much larger than the part where it falls into this passage. This resembles the nostrils; for, as the nostrils have a passage into the pharynx and into the external air, so this tube has a very small and narrow passage, like a passage out for the wind. That to the fore part of the uterus is wide and broad, as the nostrils are to the external air between the mouth and the pharynx. So women have a larger passage to the fore part of the uterus, and wider than the external passage.