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LABOR, in physiology, is the act by which a female of the genus mammalia brings one of her own species into the world. When the foetus has remained its due time in the womb, and is in a condition to carry on a separate existence, it is extruded from its place of confinement, in order to live the life which belongs to its species, independently of the mother. The womb having reached its maximum of growth with the increasing size of the foetus, its peculiar irritability excites in it the power of contraction ; it thereby narrows the space within, and pushes out the mature foetus. The period of gestation is very different in different animals, but, in each particular species, it is fixed with much precision. In the womb, the corporeal frame of man commences existence as an embryo, after further developement, appears as a foetus, then as an immature, and, finally, a mature child. With its growth and increasing size, the membranes which envelope it enlarge, the womb also expanding to give room for it. At the end of the 39fch or the beginning of the 40th week, the child has reached its perfect state, and is capable of living separate from the mother; hence follows, in course, its separation from her, i. e. the birth. Contractions of the womb gradually come on, which are called, from the painful sensa tions accompanying them, laborpains These are of two kinds: first, the preliminary pangs, which begin the labor, do not last long, are not violent, and produce the feeling of a disagreeable straining or pressure. When the pregnant female is attacked by these, she is often unable to move from her place till the pang is over, after which she is often free from pain for some hours. Then follow the true laborpains; these always last longer, return sooner, and are more violent. The contractions of the womb take place in the same order as the enlargement had previously done, the upper part of it first contracting, while the mouth of the womb enlarges, and grows thin, and the vagina becomes loose and distensible. By this means the foetus, as the space within the womb is gradually narrowed, descends with a turning motion towards the opening ; the fluid contained in the membranes enveloping the foetus, as the part making the greatest resistance, is forced out, and forms a bladder, which contributes much to the gradual enlargement of the opening of the womb. It is therefore injurious to the delivery if hasty or ignorant mid wives break the membranes too soon. By repeated and violent throes, the membranes at length burst, and discharge their contents, and, some time after, the head of the child appears. As the skullbones have not yet acquired their perfect form and substance, but are attached at the crown of the head only by a strong membrane, and may be brought nearer together, the head, by the pressure which it undergoes, may be somewhat diminished in size, and squeezed into a more oblong form, so as to pass through the opening of the matrix and the pelvis, in which it is contained, and, finally, through the external parts of generation; and when this is done, the rest of the body soon follows. The act of birth or delivery is accordingly, in general, not an unnatural, dangerous, and diseased state of the system, as many timid women imagine. It is a natural process of developement, which is no more a disease than the cutting of the teeth, or the coming on of puberty, although, like them, it may give rise to important changes in the body, and to various diseases. It is true, that the process of childbirth requires a violent exertion of nature, but this is facilitated by many preparatives and helps adapted to the purpose. If the birth succeeds in the way described, it is called a natural birth. For this, it is requisite that the pelvis should be properly formed, and that the opening should should be proportioned to the pelvis, especially that the head should have the size designed by nature, proportioned to the diameter of the pelvis; also, that there should be a proper situation of the womb, in regard to the axis of the pelvis, and a proper position of the foetus, namely, the head down, the back of the head in front, and towards the opening of the womb, so as to appear first at birth; and, finally, that the external parts of generation should be in a natural state. An easy birth takes place without any excessive strainings, and in due season. A difficult birth proceeds naturally, but is joined with great efforts and pangs, and occupies a long timeover six or eight hours. The cause of it is sometimes the stiffness of the fibres of the mother, her advanced years, the disproportionate size of the child's head, and various other causes. Nature, however, finishes even these births; and women in labor ought not to be immediately dejected and impatient, on account of these difficulties. An unnatural (or properly an irregular) birth is one in which one or more of the abovementioned requisites to a natural birth are wanting. An artificial birth is that which is accomplished by the help of art, with instruments or the hands of the midwife. Premature birth is one which happens some weeks before the usual time, namely, after the seventh, and before the end of the ninth month. Though nature has assigned the period of 40 weeks for the full maturing of the foetus, it sometimes attains, some weeks before this period has elapsed, such a growth that it may be preserved alive, in some cases, after its separation from the mother. That it has not reached its mature state is determined by various indications. Such a child, for instance, does not cry like full grown infants, but only utters a faint sound, sleeps constantly, and must be kept constantly warm, otherwise its hands and feet immediately become chilled. Besides this, in a premature childmore or less, according as it is more or less prematurethe skin over the whole body is red, often, indeed, blue, covered with a line, long, woolly hair, especially on the sides of the face, and on the back; the fontanel of the head is large, the skullbones easily moved; the face looks old and wrinkled; the eyes are generally closed; the nails on the fingers and toes, short, tender and soft, hardly a line in length; the weight of such a child is un31*is separated from the womb before the seventh month. Such children can rarely be kept alive ; there are instances, however, of five months' children living. A curious remark is found in good writers, that a seven months' child is more likely to live than one born a month later. Late birth is a birth after the usual period of 40 weeks. As this reckoning of the time from pregnancy to birth is founded, for the most part, solely on the evidence of the mother, there is much room for mistake or deception. The question is one of much interest in medical jurisprudence, as the inquiry often arises whether a child, born more than 40 weeks after the death of the reputed father, is to be considered legitimate or not. The importance of the question and the uncertainty of the proof have occasioned a great variety of opinions among medical writers. Most of them doubt the truth of the mother's assertions about such a delayed birth, and give, as their reason, that nature confines herself to the fixed period of pregnancy; that grief, sickness, &c, cannot hinder the growth of the foetus, &c. Others maintain, on the contrary, that nature binds herself to no fixed rules; that various causes may delay the growth of the child, &c. Abortion and miscarriage take place when a foetus is brought forth so immature that it cannot live. They happen from the beginning of pregnancy to the seventh' month, but most frequently in the third month. The occasions especially, in those of a susceptible or sanguine temperament, are violent shocks of body or mind by blows, falling, dancing, cramp, passion, &c.