There is some slight difference of opinion among authorities as to the precise nature and causation of the sickness of pregnancy. Barnes, Horrocks and others regard it as physiological; but many consider it pathological; this is, for instance, the opinion of Giles. Graily Hewitt attributed it to flexion of the gravid uterus, Kaltenbach to hysteria, and Zaborsky terms it a neurosis. Whitridge Williams considers that it may be (1) reflex, or (2) neurotic (when it is allied to hysteria and amenable to suggestion), or (3) toxæmic. It really appears to lie on the borderland between healthy and diseased manifestations. It is said to be unknown to farmers and veterinary surgeons. It appears to be little known among savages; it is comparatively infrequent among women of the lower social classes, and, as Giles has found, women who habitually menstruate in a painless and normal manner suffer comparatively little from the sickness of pregnancy. We owe a valuable study of the sickness of pregnancy to Giles, who analyzed the records of 300 cases. He concluded that about one-third of the pregnant women were free from sickness throughout pregnancy, 45 per cent. were free during the first three months. When sickness occurred it began in 70 per cent. of cases in the first month, and was most frequent during the second month. The duration varied from a few days to all through. Between the ages of 20 and 25 sickness was least frequent, and there was less sickness in the third than in any other pregnancy. (This corresponds with the conclusion of Matthews Duncan that 25 is the most favorable age for pregnancy.) To some extent in agreement with Guéniot, Giles believes that the vomiting of pregnancy is "one form of manifestation of the high nervous irritability of pregnancy." This high nervous tension may overflow into other channels, into the vascular and excretory system, causing eclampsia; into the muscular system, causing chorea, or, expending itself in the brain, give rise to hysteria when mild or insanity when severe. But the vagi form a very ready channel for such overflow, and hence the frequency of sickness in pregnancy. There are thus three main factors in the causation of this phenomenon: (1) An increased nervous irritability; (2) a local source of irritation; (3) a ready efferent channel for nervous energy. (Arthur Giles, "Observations on the Etiology of the Sickness of Pregnancy," _Transactions Obstetrical Society of London_, vol. xxv, 1894.)