There is another characteristic of great significance by which the sexual impulse in women differs from that in men: the widely unlike character of the physical mechanism involved in the process of coitus. Considering how obvious this difference is, it is strange that its fundamental importance should so often be underrated. In man the process of tumescence and detumescence is simple. In women it is complex. In man we have the more or less spontaneously erectile penis, which needs but very simple conditions to secure the ejaculation which brings relief. In women we have in the clitoris a corresponding apparatus on a small scale, but behind this has developed a much more extensive mechanism, which also demands satisfaction, and requires for that satisfaction the presence of various conditions that are almost antagonistic. Naturally the more complex mechanism is the more easily disturbed. It is the difference, roughly speaking, between a lock and a key. This analogy is far from indicating all the difficulties involved. We have to imagine a lock that not only requires a key to fit it, but should only be entered at the right moment, and, under the best conditions, may only become adjusted to the key by considerable use. The fact that the man takes the more active part in coitus has increased these difficulties; the woman is too often taught to believe that the whole function is low and impure, only to be submitted to at her husband's will and for his sake, and the man has no proper knowledge of the mechanism involved and the best way of dealing with it. The grossest brutality thus may be, and not infrequently is, exercised in all innocence by an ignorant husband who simply believes that he is performing his "marital duties." For a woman to exercise this physical brutality on a man is with difficulty possible; a man's pleasurable excitement is usually the necessary condition of the woman's sexual gratification. But the reverse is not the case, and, if the man is sufficiently ignorant or sufficiently coarse-grained to be satisfied with the woman's submission, he may easily become to her, in all innocence, a cause of torture.