CHAPTER VIII.
THE FŒTUS.
The embryo first begins to be distinct about the _third_ week; is then about two lines long, weighing one to two grains; is surrounded by an amnion which lies loosely about it, and obviously proceeds from the abdominal laminæ; it presents cerebral vesicles; there is the appearance of an eye, several arteries are seen though not distinctly formed; the abdominal cavity is open for a considerable extent in front.
About the _fifth_ week the embryo becomes more consistent; the head is disproportionately large; rudimentary eyes are indicated by two black spots; the abdomen is nearly closed, though at the umbilical aperture a loop of intestine escapes; the abdominal members are present, and the cord exists in a rudimentary condition; the embryo is nearly two-thirds of an inch long and weighs about fifteen grains.
The successive changes in the development were, 1st, a germ membrane visible immediately after the bursting of the vesicle; 2d, at some point an aggregation of granules forming the cumulus of the blastoderma; 3d, the embryo developed lying at this point, as it were upon the membranes, which consist of three superimposed laminæ or layers; 4th, on the serous layer arise the organs of animal life, the brain and spinal marrow, organs of sense, skin, muscles, tendons, ligaments, cartilage, and bone; on the mucous the organs of vegetative life, the intestinal canal, lungs, liver, spleen, pancreas and other glands. The heart and vascular system arise from the vascular layer (if this can be considered a separate one).
About the second week, or perhaps the third, there is a mass of globules loosely connected together forming the _primitive streak of Von Baer_, and around this the _area vasculosa_ is developed. The globules of the primitive streak, seem next to be developed into two laminæ dorsales, which is the axis of the future embryo, and the origin of the spinal column. That portion of the fluid that separates the _chorda dorsalis_ from the lamina dorsalis is the future spinal cord, and brain. Two other lamina—_laminæ ventrales of Von Baer_—are in the mean time proceeding from the axis of the embryo, one on each side; they grow laterally and converge below the axis.
After the rudiments of organic life have been commenced in the central portion of the serous layer, a fold of its peripheral portion arches over the dorsal surface of the embryo so as to represent a sac, and is composed of two membranes; the one next to the fœtus is the _amnion_, the other is gradually separated from the amnion and joins the serous lamina of the blastoderma; this is the _false amnion_ of Pander.
The heart is formed at this early period, and although there is no septum between the ventricles, a vein may be seen entering into it, and an artery passes out which divides into four branches to be distributed and ramified in different portions of the fœtus.
The abdomen is yet an open cleft, in which (but projecting beyond it) is the heart, which is of very large dimensions; behind the heart is the liver, and under the liver the intestine which is attached by means of a distinct mesentery. At this period (three weeks) the lungs are constituted of five or six lobules, and two large glandular structures may also be discerned along the vertebral column, which are called Wolfian bodies; these anticipate the function of the kidneys. The _alantois_ is seen arising from the lower part of the intestinal canal on a little vesicle and extending so as to encircle the embryo.
During the second month the extremities are growing, and become more projecting; the body is curved and the head bent downwards; there are indications of the nostrils and a gaping oral aperture; the forehead is more swelled because of the development of the hemispheres of the brain; the spinal cord is cylindrical of nearly uniform thickness and terminating in a blunt extremity—posteriorly it is open; the _medulla oblongata_ makes a bend forwards at the top of the neck, and then ascends perpendicularly into the capacious cranium.
The first centres of ossification appear about the seventh week on the clavicle and lower jaw. At this time the kidneys and renal capsules begin to appear. The only trace of muscular fibre is in the diaphragm. The vertebral arches are not yet closed in, and the ribs appear like little streaks; the liver is very large and granular. The stomach is assuming somewhat of its normal form; the urinary bladder is enclosed, but the anus is imperforate. At this time the embryo is about an inch in length.
At two months the rudimentary organs of generation may be distinguished, but their development does not clearly show the sex. The embryo is from one and a half to two inches long and weighs near half an ounce, the head forming two-thirds of the whole.
After this period the different parts are developed with tolerable rapidity. At _ten weeks_ the embryo is about two and a half inches in length. At the end of the _third month_ it is from five to six inches long and weighs from three to four ounces. The thorax is closed at all points but is only slightly developed; the cord contains no intestines, and its spiral turns are evident; the nails are beginning to appear; the sex is distinct, and the skin acquires some consistence. At the fourth month the fœtus is six to eight inches in length, and weighs from seven to eight ounces. A fœtus born at this period might live for an hour or two. At _five months_ the length of the body, including head and feet, is from eight to ten inches, and weighs from eight to eleven ounces; at _six months_ the weight is about one pound, and the length is eleven to twelve and a half inches.
At _seven months_ the fœtus is from twelve and a half to fourteen inches long and weighs from three to five pounds. The hands and feet, including the nails, are developed; all its parts are tolerably firm, and their respective dimensions better proportioned than formerly. The scrotum usually contains one or both testicles, they having descended through the inguinal ring, from their original place near the vertebral column; the eyelids are partly open; the skin is very red and covered with down. Many children live and are reared that are born at seven months.
The length of a fœtus born at term is eighteen or nineteen inches, though the utmost limit is more than two feet. The usual weight is from six to seven pounds; children have been born, however, that were as much as eighteen pounds in weight. I suspect that this will never occur unless the term has been extended beyond the usual period.
AT TERM the fœtus that is twenty inches long will generally measure ten and a half to eleven inches from the crown to the umbilicus. The different parts are well developed and partly covered by a smegma called the _vernix caseosa_; the head has attained the proper hardness, and the scrotum usually contain the testicles. In female children the nymphæ are generally covered entirely by the labia, the breasts project, and in both sexes contain frequently a milky fluid.
As soon as the child that has been carried the full time is born it usually cries, opens its eyes, and makes some struggling motions with its limbs; it soon passes urine and feces, and readily takes the breast.
With occasional exceptions the position of the child is unaltered from an early period of pregnancy to its termination, whether the head be upwards or downwards. The arms are generally folded over the chest, the knees drawn up to the abdomen; the back curved, and the head bent upon the chest, so as to occupy as little space as possible. In ordinary cases the head is directed downwards, and the face looks obliquely, so that in the first and second position the back of the fœtus is turned partly forwards, and the belly in the third and fourth. We are enabled in many cases to ascertain the position of the fœtus in the uterus before labor has commenced, by means of the stethescope, by noting whether the pulsation is felt on one side or the other of the abdomen and observing whether it is heard clearly or not.
The longitudinal diameter of the head is from 4 to 4½ inches, the transverse from 3½ to 4, the vertical 3 to 3¾ inches. The transverse diameter of the shoulders and thorax is 4¾ to 5½; the widest diameter of the hips 4 to 5 inches. In general the measurements are a little less in the female than in the male.
The head of the fœtus is large, and as it is less compressible at term than other portions it merits a particular description; we should be acquainted with all its characters, that we may recognize them and thereby determine the position during labor.
The fœtal head is ovoidal in form, the large extremity being posterior. Several bones enter into the formation of the cranium; they are, 1st, the _frontal_ bone forming the forehead; in the fœtus even at term it is usually divided; 2d, the two _parietal_ bones, one on each side of the head, meeting on the median line at the top of the head; they help to form the vault of the cranium; 3d, the occipital bone, forming the posterior and part of the base of the skull; and 4th, the temporal bones, one on the right and one on the left side below the parietal, completing the lateral portions of the cranium and contributing to form the base of it. The cranial bones are not united to each other by sutures as they are in the adult, but are separated, the parietal bones especially, by membranous intervals, the intervals being larger in some children than in others. These intervals, or _sutures_ and _fontanelles_, must be carefully studied.
The _sagittal suture_ is the antero-posterior one, and extends from the root of the nose to the occipital bone. It is formed in front by the interval that separates the frontal bone into two halves, and superiorly by that between the two parietals. There is a suture which crosses this, called the transverse or _coronal suture_, which is formed by the space existing between the frontal and parietal bones. When the sagittal suture arrives at the superior angle of the occipital bone, it seems to part and give rise to two oblique lateral sutures which are called _lambdoidal_; these are formed by the posterior borders of the parietal bones and the superior one of the occipital.
Just at the point where the coronal and the lambdoidal sutures join the sagittal one, two membranous spaces, larger than those just described, are found; these have received the name of _fontanelles_.
In cases of head presentation during labor, one or the other of the fontanelles may be felt by the attending practitioner, and this indicates to him the position of the head and the presentation.
The _anterior fontanelle_ presents an extensive surface at the place where the transverse crosses the sagittal suture. It is lozenge-shaped, and is bounded by four bony angles.
The _posterior fontanelle_ is formed by the union of the two lambdoidal sutures with the termination of the sagittal suture. It is smaller than the anterior one, and is of a triangular form. It is bounded by the occipital bone and the angles of the parietal bones. During labor the bones may overlap each other so that the sutures cannot be felt, but the prominences of the bony margins will aid the diagnosis.
THE PHYSIOLOGY OF FŒTAL LIFE.
The ovule, after it arrives in the uterine cavity, comes in contact at all points with the mucous membrane of the uterus. Its nutrition at first is organic by superficial imbibition; afterwards, probably the villi of the chorion imbibe the fluids there secreted, and transmit them into the space between the chorion and the amnion, thence it transcends through the walls of the amnion, and a portion is conveyed into the fœtus through the umbilical vesicle. After the placenta is formed there may still be some absorption of some of the nutritive matters contained in the liquor amnii through the skin of the fœtus, but its growth is principally maintained by an assimilation of that which the radicles of the umbilical vessels take up in the placenta. By means of the extensive contact existing between the vascular apparatus of the two placentas, a transudation probably takes place of some part of the maternal blood, which is absorbed and mingled with the fœtal blood, and furnishes some of the nutritive material.
When mingled with the fœtal blood, the nutritive elements supplied by the mother are devoted to the development of the organs. It is supposed, however, that they undergo changes in the large liver of the fœtus and in its intestines.
There is no true respiration in the uterine cavity, but one function of the placenta is to renew the blood of the fœtus from that of the mother, in about the same way that the blood of fishes is ærated by the water passing through the gills.
Whether in the earlier months absorption is carried on by the surface alone, or whether a portion of the liquor amnii finds its way to the stomach is difficult to decide, but, without doubt, a certain amount of digestion is carried on.
The CIRCULATION of the blood in the fœtus cannot be understood without referring to certain anatomical peculiarities that do not exist in the adult. These characteristics depend on the absence of respiration, and disappear when it is established.
1st. The septum between the auricles of the heart is imperfect, having in its center a valvular oval aperture called the _foramen ovale_.
2d. The pulmonary artery, soon after its origin, gives off a branch, the _ductus arteriosis_, which enters the aorta just below the arch. The pulmonary arteries are very small.
3d. The _umbilical artery_ in the _fœtus_ is a large vascular trunk, which is nearly obliterated in after life. The two umbilical arteries run forwards and inwards along the lateral and superior parts of the bladder, then curve forwards to the abdominal wall, along which they ascend to the umbilicus, then pass along the cord to the placenta.
4th. The fœtus further differs from the adult in having an umbilical vein, which comes from the placental tissue, traverses the length of the cord, passes through the umbilical ring, is mostly distributed to the liver, but has a supplemental vein situated at the thick edge of the liver, and leading to the vena cava ascendens, called the _ductus venosis_.
The general effect of all these peculiarities is to render the heart virtually a single one; to provide for the quiescent state of the lungs, and to modify the distribution of fresh blood.
THE COURSE OF THE BLOOD IN THE FŒTUS IS AS FOLLOWS: The blood circulating in the umbilical vein is, on entering the fœtus, a part of it discharged through the _ductus venosis_ into the vena cava; another part is distributed to the liver, and is brought to the vena cava by the hepatic veins, and then mingles also with that from the inferior extremities, and then with that from the upper extremities as it passes into the right auricle. A part of this is transmitted through the right ventricle, and thence (except a supply for the nourishment of the lungs) through the ductus arteriosis into the descending aorta. A second and larger part passes through the foramen ovale into the left auricle, then into the left ventricle and arch of the aorta, the branches of which supply the head and upper extremities. The continued stream passes into the descending aorta, mixing with that already described. The whole now descends to the lower part of the aorta, where a portion is sent to the inferior extremities, but a larger part is drawn into the umbilical arteries, and is carried by them into the placenta.
After birth remarkable changes take place. Something in the circumstances in which the child is placed stimulates respiration and crying, by which means the lungs are inflated, and space is afforded to the pulmonary circulation, which supercedes the use of the foramen ovale and ductus arteriosis; the blood from the lower extremities cannot pass through the umbilical arteries, and does pass through the ascending cava into the right auricle and ventricle, then into the lungs, where it undergoes analagous changes to those effected in the placenta, and is distributed to the body generally. The fœtal openings are generally obliterated in the course of a week, though the foramen ovale, or the ductus venosis, may continue pervious for two or three weeks; but soon the ductus venosis and the umbilical arteries are obliterated and the adult circulation established.