Part 3 (1/2)
The Uterus.--This is a hollow, pear-shaped organ, located in the median line, just behind the bladder, between it and the r.e.c.t.u.m. It is supported in place by various ligaments and by the juxtaposition of other organs. Its larger end is directed upward, and communicates upon each side with a very narrow tube which is prolonged outward on either side until it nearly touches the ovary of the same side. Its lower and smaller end fills the internal extremity of the pa.s.sage previously described as the v.a.g.i.n.a. When an ovum is matured, it escapes from the ovary into the narrow tube referred to, called the _Fallopian tube_, and pa.s.ses down into the cavity of the uterus. If fecundation does not occur, it is expelled or absorbed after six to twelve or fourteen days.
If copulation occurs, however, zoosperms are brought into the cavity of the uterus, and, coming in contact with the ovum, fecundate it. This is _conception_. When the natural process is allowed to proceed, development occurs.
Uterine Gestation.--This is the term applied to the process last referred to. We shall not attempt to describe in detail this most wonderful and intricate of all living processes; but will sketch only the chief points, leaving the reader who would obtain a more complete knowledge of the subject to consult any one of the numerous physiological and obstetrical works which deal with it in a very exhaustive manner.
As soon as the ovum is impregnated by the male element, it begins a process of symmetrical division. The first division produces two cells out of the single one which first existed. By the next division, four segments are produced; then eight, sixteen, etc. While this process is going on, the ovum becomes adherent to the internal wall of the uterus, and is soon enveloped by its mucous membrane, which grows up about and incloses it.
The Primitive Trace.--When the process of segmentation has advanced to a certain point, the cells are aggregated together in a compact layer at the surface. Soon a straight line appears upon this layer, which is called the _primitive trace_. This delicate line becomes the basis for the spinal column; and upon and about it the whole individual is developed by an intricate process of folding, dividing, and reduplication of the layer of cells. One end of the line becomes the head, and the other becomes the tail. Even man has a caudal appendage at an early stage of his existence. After a further lapse of time, little excrescences, buds, or ”pads,” appear in the proper positions to represent the arms and legs. After further development the ends split up into fingers and toes, and by the continued development of the parts, perfect arms and legs are formed.
Curious Relation to Lower Animals.--It is a very remarkable fact that in the lower animals we have numerous examples in which the permanent condition of the individual is the same as some one of the stages through which man pa.s.ses in the process of development. The same author previously quoted makes the following interesting statements:--
”The webbed feet of the seal and ornithorhynchus typify the period when the hands and feet of the human embryo are as yet only partly subdivided into fingers and toes. Indeed, it is not uncommon for the 'web' to persist to some extent between the toes of adults; and occasionally children are born with two or more fingers or toes united to their tips.
”With the seal and the walrus, the limbs are protruded but little beyond the wrist and ankle. With the ordinary quadrupeds, the knee and elbow are visible. The cats, the lemurs, and the monkeys form a series in which the limbs are successively freed from the trunk, and in the highest apes they are capable of nearly the same movements as the human arm and leg, which, in their development, pa.s.sed through all these stages.”
Simplicity of Early Structures.--The first structures formed are exceedingly simple in form. It is only by slow degrees that the great complicity which characterizes many organs is finally attained. For example, the heart is at first only a straight tube. By enlargement and the formation of longitudinal and transverse part.i.tions, the fully developed organ is finally produced. The stomach and intestines are also at first but a simple straight tube. The stomach and large intestine are formed by dilatation; and by a growth of the tube in length while the ends are confined, the small intestines are formed. The other internal organs are successively developed by similar processes.
The Stages of Growth.--At first insignificant in size--a simple cell, the embryonic human being steadily increases in size, gradually approximating more and more closely to the human form, until, at the end of about nine calendar months or ten lunar months, the new individual is prepared to enter the world and begin a more independent course of life. The following condensation of a summary quoted by Dr.
Austin Flint, Jr., will give an idea of the size of the developing being at different periods, and the rate of progress:--
At the end of the third week, the embryon is a little less than one-fourth of an inch in length.
At the end of the seventh week, it is three-fourths of an inch long.
The liver, lungs, and other internal organs are partially formed.
At the eighth week, it is about one inch in length. It begins to look some like a human being, but it is impossible to determine the s.e.x.
At the third month, the embryon has attained the length of two to two and one-half inches. Its weight is about one ounce.
At the end of the fourth month, the embryon is called a fetus. It is from four to five inches long, and weighs five ounces.
At the fifth month, the fetus is nearly a foot long, and weighs about half a pound.
At the sixth month, the average length of the fetus is about thirteen inches, and its weight one and a half to two pounds. If born, life could continue a few minutes.
At the seventh month, the fetus is from fourteen to fifteen inches long, and weighs two to three pounds. It is now viable (may live if born).
At the eighth month, the length of the fetus is from fifteen to sixteen inches, and its weight from three to four pounds.
At the ninth month, the fetus is about seventeen inches long, and weighs from five to six pounds.
At birth, the infant weighs a little more than seven pounds, the usual range being from four to ten pounds, though these limits are sometimes exceeded.
Duration of Gestation.--The length of time required for the development of a human being is usually reckoned as about forty weeks. A more precise statement places it at about two hundred and seventy-eight days. This limit is often varied from. Cases have occurred in which a much longer time has been required, and numberless cases have occurred in which human beings have been born several weeks before the expiration of the usual time, as stated. There is some uncertainty respecting the exact length of the period of gestation, which grows out of the difficulty of determining, in many cases, the exact time when conception takes place.
Uterine Life.--The uterine life of the new individual begins with the impregnation of the ovum, which occurs the instant it is brought in contact with the zoosperms of the male. While in the uterus, the young life is supported wholly by the mother. She is obliged to provide not only for her own sustenance, but for the maintenance of her child. And she must not only eat for it, but breathe for it as well, since it requires a constant and adequate supply of oxygen before birth as much as afterward.
How the Unborn Infant Breathes.--Oxygen and nutriment are both supplied to it through the medium of an organ called the _placenta_, which is a spongy growth composed almost entirely of blood-vessels, and is developed upon the inner wall of the uterus, at the point at which the ovum attaches itself after fecundation. The growing fetus is connected with this vascular organ by means of a sort of cable, called the _umbilical cord_. The cord is almost entirely composed of blood-vessels which convey the blood of the fetus to the placenta and return it again.
The fetal blood does not mix with that of the mother, but receives oxygen and nourishment from it by absorption through the thin walls which alone separate it from the mother's blood.
The umbilical cord contains no nerves, as there is no nervous connection between the mother and the child. The only way in which the child can be influenced by the mother is through the medium of the blood, to changes in which it is very susceptible, as we shall see more clearly hereafter.
The cord is attached to the body of the child at the point called the _navel_, being cut off at birth by the _accoucheur_. With the placenta, it is expelled soon after the birth of the child, and const.i.tutes the shapeless ma.s.s familiarly known as the _after-birth_, by the retention of which the most serious trouble is occasionally caused.