CHAPTER VIII.
FORM AND SIZE OF THE FŒTUS AT FULL TERM.
SIZE AND FORM OF THE BODY.
The average length of the Fœtus, at full term, is about twelve inches from the head to the breech, and about eighteen inches from the head to the feet. Its weight varies from five to eight pounds, perhaps averaging about six, though some have been born weighing only three pounds, or less, and some even as high as twelve. The breadth across the shoulders is about four inches, and the same across the hips, but both are so easily compressed that during delivery they only measure about three inches, or three and a half at most.
SIZE AND FORM OF THE HEAD.
The head is the most important part, because it is the largest, and usually present first. It is therefore necessary to describe it fully, and with special reference to its importance, in the early stages of labor, as the part by which the position is usually determined.
PLATE XXIV.
The head is generally divided into the Cranium, or that part which contains the brain, and the Face.
_The Bones of the Cranium._--These are seven in number, viz., two _Frontal_ Bones, or those forming the forehead; 1, 1, Figs. 1 and 2, (Plate XXIV.)--Two _Parietal_ bones, or those forming the sides of the head; 2, 2, Figs. 1 and 2.--The _Occipital_ bone, or that forming the back of the head; 3, 3, Figs. 1 and 2.--And two _Temporal_ bones, which lie over and between the ear and the eye; 4, Fig. 2.
_The Bones of the Face._--These are five in number, viz., two _Superior Maxillary_, or upper jaw bones; 5, Fig. 2.--Two _Malar_, or cheek bones; 6, Fig. 2.--And one _Inferior Maxillary_, or lower jaw bone; 7, Fig. 2.
[Illustration: THE FŒTAL HEAD.
Plate XXIV.
The head is generally divided into the Cranium, or that part which contains the brain, and the Face.]
The bones of the cranium are not closed together, as they are in the adult, but are separated to a considerable distance, in certain parts, and connected by a strong membrane. These membranous spaces are called _Sutures_ and _Fontanelles_, and a knowledge of them is absolutely necessary, as a means of ascertaining the position of the head.
_The Sutures._--The first of these spaces, which extends from the lowest part of the middle of the forehead to the occipital bone, is called the _Sagittal Suture_, or antero posterior Suture: A, B, C, Figs. 1 and 2. It separates the two frontal, and the two parietal bones. The spaces between the two frontal and the two parietal bones are called the _Frontal Parietal Sutures_, _o. o._ Fig. 1; and those between the two parietal bones and the occipital, _l. l._ Figs. 1 and 2, are called the _Lambdoidal Sutures_.
_The Fontanelles._--When the different Sutures meet at a point, the membranous space is greater than at other parts, and is called a Fontanelle. Thus when the two Frontal Parietal Sutures meet the Sagittal Suture, at B, Figs. 1 and 2, there is quite a large diamond shaped space between the different bones, filled up with membrane. This is called the _Anterior Fontanelle_, or bregma. Where the Sagittal Suture joins the two Lambdoidal there is another space, not so large, and different in shape, being triangular; this is called the _Posterior Fontanelle_, _c._ Figs. 1 and 2. And where each of the temporal bones joins the parietal there are two other spaces, called the _Temporal Fontanelles_, _t._ Fig. 2, which are also irregularly diamond shaped, but not nearly so large as the anterior fontanelle.
It is evident that if a person can distinguish these Fontanelles, when he touches them with his finger, he can tell what part of the head is presenting, and hence their use.
These spaces allow of the bones lapping over each other, during delivery, and thus the head is made smaller. They do not disappear till sometime after birth, and in very young children the brain may be felt, and seen to work, at the anterior fontanelle. Eventually however, the bones come close together, and are joined by a curious kind of dovetailing. The two frontal bones however, completely coalesce, and form but one, in the adult.
Sometimes the bones will be very perfectly formed, and the fontanelles nearly filled up, before birth, and then the head cannot be crushed much smaller, and so the labor becomes both painful and difficult. This is usually called an ossified, or solid head.
_Diameters of the Head._--The diameters of the head are the distances between its most prominent points. They are necessary to be known before we can judge as to the possibility of its passing the straits of the Pelvis, in the various positions.
Generally there are reckoned _ten_ diameters, and they are represented in the following Plate:--
[Illustration: PLATE XXV.
_Diameters of the Head._
1. The _Occipito Mental_, or from the back of the head to the chin, M, O, Fig. 1, five inches.
2. The _Mento Bregmatic_, or from the chin to the anterior fontanelle at the top of the head, M, O, Fig. 1, four inches.
3. The _Occipito Frontal_, or from the back of the head to the top of the forehead, O, F, Fig. 1, four inches.
4. The _Trachelo Occipital_, or from the throat to the back of the head, T, O, Fig. 1, four inches.
5. The _Sub Occipito Bregmatic_, or from the nape of the neck to the top of the head, S, B, Fig. 1, three inches and a half.
6. The _Trachelo Bregmatic_, or from the top of the throat to the top of the head, T, B, Fig. 1, about three inches and a half.
7. The _Trachelo Frontal_, or from the top of the throat to the top of the forehead, T, F, Fig. 1, about three inches.
8. The _Sub Occipito Frontal_, or from the nape of the neck to the top of the forehead, S, F, Fig. 1, about three inches.
9. The _Bi Parietal_, or across the head from one side to the other at the middle of the parietal bones, the widest part, B, P, Fig. 2, three inches and a quarter to three and a half.
10. The _Bi Temporal_, or across the head from one temporal bone to the other, B, T, Fig. 2, about two inches and a half.]
The necessity for knowing these diameters will be obvious. It is only by being acquainted with them, and with the straits of the Pelvis, already described, that we can tell whether the child can pass or not in certain positions, and how we must change its position, when possible, to give relief.
On comparing the diameters of the head with those of the Pelvis, it will be seen that some of them correspond in size while others do not, so that in one position delivery can take place spontaneously, while in another it will be difficult, and in some nearly impossible, without assistance. Thus, for instance, if the head should present by the occipito mental diameter, (O, M, Fig. 1,) it evidently could not pass while in that position, because by this diameter it is five inches in width, and the greatest diameter of the Pelvis is only about four inches and a half. Its position must therefore be changed, and the accoucheur must know how to change it with advantage.
ATTITUDE OF THE FŒTUS AT FULL TERM.
The attitude of the Fœtus is represented in the following Plate:--
[Illustration: PLATE XXVI.
_Attitude of the Fœtus._
The arms, it will be seen, are crossed on the breast, upon which the chin is also bent; the thighs are close together, and brought against the Abdomen; the legs are close bent on the Thighs, and the feet are turned up against the front of the legs, the whole body being curved forward.]
The position in which the Fœtus most usually lies in the Womb has already been shown, particularly in Plate XII, and in Plate XVI. The head is downwards, and the back part of it turned to the mother's left side. What is the cause of this almost universal position is not certainly known. It was formerly thought to depend on the head being heavier than the other parts, and thus sinking down; but this supposition has been shown to be entirely without foundation. M. P. Dubois has lately advanced the notion that it depends on an instinctive feeling in the Fœtus itself, which directs it to take that position by which it can most easily make its exit. This opinion appears very reasonable, and is apparently well founded. It is well known that the Fœtus is susceptible of various impressions while in the Womb, and impelled by unerring instinct to take the breast immediately it is born; nay, it has even been known to suck the finger of the assistant, in cases of face presentation, even before birth! We can readily believe, therefore, that it is directed to place itself in the Womb, in the best position, the same as it is directed to take the finger in its mouth.
In the young of many of the lower animals this is also strikingly exemplified. The young duck in the shell taps with its little beak against the part that is to be broken, and rushes into the water even with a part of the shell still on its back. The young oppossums, who are born imperfect from the Womb, shelter themselves, immediately they come into the world, in the pouch on the mother's breast, and fasten themselves to the mammæ till they are more perfectly grown.
It must be remarked however, that the head does not _always_ present first, though it usually does so. Occasionally we have the breech present, and still rarer even other parts; but these are merely exceptional deviations, the causes of which are unknown. Out of every sixteen children born, _fifteen_ usually come head first. This however, will be shown better further on.