The justification for using the term “tumescence,” which I here propose, is to be found in the fact that vascular congestion, more especially of the parts related to generation, is an essential preliminary to acute sexual desire.  This is clearly brought out in Heape’s careful study of the “sexual season” in mammals.  Heape distinguishes between the “pro-estrum,” or preliminary period of congestion, in female animals and the immediately following “estrus,” or period of desire.  The latter period is the result of the former, and, among the lower animals at all events, intercourse only takes place during the estrus, not during the pro-estrum.  Tumescence must thus be obtained before desire can become acute, and courtship runs pari passu with physiological processes.  “Normal estrus,” Heape states, “occurs in conjunction with certain changes in the uterine tissue, and this is accompanied by congestion and stimulation or irritation of the copulatory organs....  Congestion is invariably present and is an essential condition....  The first sign of pro-estrum noticed in the lower mammals is a swollen and congested vulva and a general restlessness, excitement, or uneasiness.  There are other signs familiar to breeders of various mammals, such as the congested conjunctiva of the rabbit’s eye and the drooping ears of the pig.  Many monkeys exhibit congestion of the face and nipples, as well as of the buttocks, thighs, and neighboring parts; sometimes they are congested to a very marked extent, and in some species a swelling, occasionally prodigious, of the soft tissues round the anal and generative openings, which is also at the time brilliantly congested, indicates the progress of the pro-estrum....  The growth of the stroma-tissue [in the uterus of monkeys during the pro-estrum] is rapidly followed by an increase in the number and size of the vessels of the stroma; the whole becomes richly supplied with blood, and the surface is flushed and highly vascular.  This process goes on until the whole of the internal stroma becomes tense and brilliantly injected with blood....  In all essential points the menstruation or pro-estrum of the human female is identical with that of monkeys....  Estrus is possible only after the changes due to pro-estrum have taken place in the uterus.  A wave of disturbance, at first evident in the external generative organs, extends to the uterus, and after the various phases of pro-estrum have been gone through in that organ, and the excitement there is subsiding, it would seem as if the external organs gain renewed stimulus, and it is then that estrus takes place....  In all animals which have been investigated coition is not allowed by the female until some time after the swelling and congestion of the vulva and surrounding tissue are first demonstrated, and in those animals which suffer from a considerable discharge of blood the main portion of that discharge, if not the whole of it, will be evacuated before sexual intercourse