DIVISIONS OF ANTENATAL LIFE 7
ledge can judge how preparations are progressing. The accompanying scheme of the divisions will serve, taken in conjunction with the descriptive notes, to give to the mind a somewhat clearer conception of the chronology of the period of preparation for the great events of postnatal life ; it ^vill take the place of the prologue in explaining the action of the to be enacted drama (Fig. 1).
In constructing the scheme 1 have employed the " space-for- time " method introduced into medical case-recording by Mr. Jonathan Hutchinson, and described by him in 1896 {Arch. Surg., 1896, voL vii. p. 199). By this plan, all periods of time are represented in the schedule by equal extents of space, no time is left out, and the whole duration of the antenatal epoch, with its various events in their proper places, is brought correctly before the eye. Each interspace in the scheme represents a week ; and as pregnancy lasts normally for forty weeks, there are forty interspaces intervening between its beginning and end ; l3ut as the month following birth is much influenced by what has happened before birth, and is, indeed, a transition period between antenatal and postnatal life, it also has found a place in the scheme, and has four interspaces. Above the neonatal period are to be imagined the many spaces indicating the many weeks of postnatal existence. The great physiological event of neonatal existence is the adaptation of the organism to its new environ- ment ; the foetus is suddenly brought into surroundings which demand the functional awakening of several organs which have in intrauterine life been almost if not quite dormant, and structures which have been active have to atrophy, be absorbed, or be utilised for other than their antenatal purposes. Extrauterine life is linked, as it were, to intra- uterine by this short period of the new-born infant.
Immediately before the neonatal period (below it, therefore, in the schedule), and separated from it by the event of birth (indicated in the schedule by a thick black line), is ihefmtal epoch. This occupies by far the largest part of pregnancy ; without reckoning the neofoetal period, it extends from the eighth to the fortieth week, or thirty-two weeks. During its progress the organism shows its vitality chiefly by growth along lines which have been already definitely laid down. In this respect it resembles the postnatal periods of infancy and youth. It is true that the intrauterine environment has very dis- tinctive and peculiar characters — the unborn infant exists in a fluid medium of practically constant temperature, it is protected from traumatism by the maternal structures, and it is shut in from the light ; further, the fcetus has several of its organs almost inactive, and its most important and most active organ, the placenta, is extra- corporeal ; nevertheless, the chief phenomenon of foetal life is growth, rapid and continuous, along lines already indicated. Within seven (calendar) months, which is the length, roughly speaking, of foetal life in the human subject, the organism increases from a structure 1 in. in length to one measuring 20 in., and its increase in weight is from 1 oz. to 7 or 8 lbs.
During the emhryonic period of antenatal life, which may be said to begin with the laying down of the first rudiments of the embryo
