Reproduction


Reproduction


    (* It has been thought advisable to introduce this short chapter for, although it deals with normal phenomena yet all the process of reproduction is for one reason or another so guarded from discussion that there is often gross ignorance of matters of vital importance. This ignorance must tend to ill in the community and so in a manual of health a chapter such as this can claim a place.)

Throughout the animal kingdom we find the welfare of the individual subordinated to that of the species. The crowning act of an animal’s life is the reproduction of a new individual fitted in all respects to take the place of the parent organism and so maintain the race on earth.

In the case of the lowliest unicellular organisms, which reproduce themselves only by fission, we cannot rightly speak of dearth from natural causes. One amoeba divides into two new individuals similar to it in every way, and these in their turn divide again. Hence the amoebae have with some right been spoken of as immortal.

As we go higher in the scale, we meet with more highly differentiated organisms, consisting of cell colonies each member of which has its own appointed task to fulfill and here we find that the office of reproduction also is confined to one cell or group of cells- called in the female the ovary, and in the male the testicle. The immortality of the amoeba has been transmitted to this group of cells. These reproductive organs might be eliminated entirely and the power of the body as an organism to maintain its individual existence not be seriously interfered with.

In the sexual mode of reproduction, which obtains in man and all the higher animals, the conjunction of two cells is required. One cell is the female element called the ovum and is developed in the ovary; the other, the male, cell is called the spermatozoon and is developed in the testicle. These two cells unite by the penetration of the spermatozoon, which is motile, into the ovum to form a new cell which, thereupon, begins to grow rapidly and produce an organism that in all of its manifold peculiarities of structure and function is essentially a replica of its parents.

The sexual fertilization of the female cell or ovum is supposed to take place after its extrusion from the ovary and shortly after its entrance into the Fallopian tube, which leads from the ovary to the uterus or womb. By the act of coitus the motile spermatozoa of the male are deposited at the mouth of the uterus, whence they make their way towards the Fallopian tubes. The ovum unites with a spermatozoon, and under normal circumstances, with only one.

The period of active sexual life, during which the individual is capable of begetting or bearing children begins in both sexes about the age of fourteen to sixteen, known as the age of puberty., In women, the beginning of this period is marked by the onset of menstruation, which is a phenomenon dependent upon periodical activity in the ovaries, and takes the form of a flow of mucus and blood from the genital organs. It lasts from three to five days and recurs every four weeks-hence it is known as the monthly period, menses, menstruation, or catamenia. The interval is not absolutely regular, and shows many individual variations within limits which may be placed at twenty to thirty- five days. The quantity of blood lost is also subject to individual variations.

Menstruation is associated with ovulation, which latter consists in the discharge of an ovum from the ovary into the Fallopian tube whence it is conducted to the uterus. If the ovum be not fertilized, it is expelled with the blood and products of disintegration of the uterine mucous membrane at each menstrual period. If, however, it be fertilised while in the Fallopian tube, a considerable thickening of the uterine mucous membrane takes place, and when the fertilized ovum reaches the uterus it becomes imbedded in this thickened mucous membrane which grows round it and by means of its blood-vessels affords it nourishment.

In animals which have a rutting season, ovulation is also accompanied by a flow of blood and mucus from the genital organs, and it is during this period, which corresponds to the menstrual period that impregnation is effected. In the human species, impregnation may be effected at any time, and the union of spermatozoon with ovum may occur in the uterus, Fallopian tube, or even on the surface of the ovary.

Certain premonitory symptoms usually precede each appearance of the menses, such as pains in the back or head or a general feeling of discomfort, although in some cases these symptoms are absent. When these premonitory symptoms are unusually painful or serious, and the flow is difficult or irregular, the condition is designated as dysmenorrhoea.

Absence of the menstrual flow is designated as a condition of amenorrhoea, and occurs during pregnancy and also generally during lactation. Menstruation ceases altogether between the ages of forty-five and fifty. After this time, which is known as the natural menopause, climacteric, or change of life, the woman is no longer capable of bearing children. The change is sometimes abrupt, sometimes very gradual, being preceded by irregularity in menstruation, and it is not unfrequently associated with psychical disturbances. If, at any time, during sexual life the ovaries are completely removed by surgical operation, menstruation is brought to a close, this condition being designated as the artificial menopause.

At about the eighth week after impregnation the formation of the placenta takes place. It is through the medium of the placenta that the developing animal obtains all the nutrient material it requires-both oxygen and combustible food stuffs. The placenta, at the same time, serves as an excretory organ for the foetus, and is therefore alimentary, respiratory, and excretory.

While the ovum is undergoing its wonderful developments in which a complete human being is being formed out of a single cell by division and differentiation, the uterus becomes very much enlarged, and its walls thickened by new growth of muscular tissue. At the end of nine months from the date of impregnation, the development of the foetus is complete and parturition takes place. This consists in the expulsion of the foetus by muscular contraction of the uterus.

Parturition, or labour, is divided into three stages. In the first stage the contractions of the uterus which are not unfrequently painful and hence spoken of as “pains”, are devoted to dilating the mouth of the uterus. When this is fully dilated the uterine contractions change in character, becoming more prolonged and are accompanied by strong contractions of the abdominal muscles which force the child out through the vagina. A short time after the birth of the child, the “pains” recommence and expel the placenta with the decidua and foetal membranes. After birth the enlarged uterus rapidly diminishes in size in consequence of the atrophy and disintegration of the newly formed muscular tissues, and this involution is complete at the end of three months.

The young child at birth is not independent, but relies for many years for nourishment and protection on the parents. For the first nine months of the child’ existence under normal circumstances it is nourished almost entirely on the secretion of the mammary glands of the mother. Although milk may be expressed from the breast as early as the third -month of pregnancy, the active secretion begins shortly before or immediately after parturition. The first milk that is secreted, which is called colostrum, differs markedly from normal milk. The active secretion of ordinary milk sets in on the second or third day after delivery.

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