FŒTAL DEVELOPMENT.
Thelast Chapter brought us up to the point of conception, or the first commencement of the new being. The next step is to exhibit its various stages of development, and to show how it is nourished and maintained in its proper position. This is requisite in order to understand the origin of many diseases and accidents which occur during gestation, and also to explain the various signs by which it is determined whether a female is pregnant or not.
The development of a perfectly formed human being from the egg in which it originates, is one of the most astonishing phenomena that can come under our observation, and is eminently deserving the study of every rational being. The present explanation of it must necessarily be brief, being merely intended to make the main subject more clear.
It has already been remarked that it is uncertain whether the ovum is impregnated before it is brought into the Uterus, or after it arrives there; but be that as it may, nothing has yet been discovered in the Womb till several days after conception. Some physiologists tell us that the rudiment of the new being may be found there about thesixthday; but others again assure us that it cannot be found before thetwelfth, at which period our explanation of its development will commence.
At the twelfth daythe ovum is about the size of a large pea, it is composed of a vesicle containing a thick fluid, called thegerm, which corresponds to the opaque substance seen in the white of a fecundated bird's egg, and of a yellowish substance, in which it floats, called thevitellus, which answers to the yelk. The whole being surrounded by two membranous coverings, the outer one called thechorion, and the inner one theamnion. Between these is a gelatinous substance, and within the amnion is a fluid, called the liquor amnii. The two membranes, the liquor amnii, and the inclosed ovum, are called theovulum!Immediately after conception the uterus begins to secrete, from its inner walls, another membrane, very delicate, called thedecidua. This lines the whole cavity, so that when the ovulum passes out of the tube it is met by this lining which seems to prevent its entrance into the womb. The ovulum, however, presses upon it and so makes a depression, like a nest, in which it lies. This prevents its moving about, or falling to the bottom of the womb.
The weight of the entire ovulum is about one grain. The embryo commences in the germ, and may now be seen about the size of a pin's point. The vitellus removes away from it, but remains connected by a small pedicel or thread-like tube, down which it is gradually absorbed as nutriment. A small white thread, scarcely perceptible, may be seen sometimes as early as this period, being the commencement of the brain and spinal marrow. The mouth is visible also from the twelfth to the twentieth day, and frequently the eyes. These are placed at first on the side of the head, like those of quadrupeds, and move round to the front afterwards.
At twenty-five days, the embryo is about the sizeof a large ant, which it also resembles in form. It begins to have a little more consistence, and the future bones begin to resemble cartilage, or gristle. A small groove may be seen denoting the neck, which thus indicates the separation of the head from the trunk. The weight is three or four grains.
The first month, it is about the size of a Bee, and is somewhat like a small worm bent together. The arms may be seen like two little warts. They are first formed under the skin, and shoot out like buds, growing straight from the body; afterwards they become folded together, in a curious manner, upon the breast. The head is as large as the rest of the body, and upon it we can now see distinctly the eyes, like two black dots, the mouth, like a line, and also the nose. The lower extremity is lengthened out like a tail. Weight about ten grains.
The second month.Every part has now become much more developed, and the general form is that of a human being. The superior members are much more elongated, and the inferior ones begin to be distinguished, forming in the same manner as the others. The fingers are united together by a membrane, like the web on a Frog's foot. In the ribs, clavicles, and jaw bones, a few points can be seen ossified, the cartilage beginning; to harden into bone. The rudiments of the first teeth are also visible. The weight is about one drachm, and the length one inch.
At about seventy daysthe eyelids are visible, the nose becomes prominent, the mouth enlarges, and the external ear may be seen. The neck is well defined. The brain is soft and pulpy, and the heart is perfectly developed.
Every organ is originally formed without either blood or blood vessels. The circulation which afterwardstakes place in them is merely for their subsequent development. The heart is perfect in all its parts, and even has a slight motion, before the blood is found in it.
Three months.All the essential parts are well defined. The eyelids distinct, but firmly closed. The lips perfect, but drawn tightly together. The heart beats forcibly, and in the larger vessels red blood is seen. The fingers and toes are defined, and the muscles begin to be apparent. The organs of generation are remarkably prominent, but still it is somewhat difficult, at first, to distinguish the sex by these organs, notwithstanding their development, as the principal parts in both are nearly identical in form. It can, however, be ascertained by other circumstances, as the form of the head, dorsal spine, thorax, and abdomen. It now weighs about two ounces and a half, and measures four or five inches in length.
Four months.The development is remarkably increased. The brain and spinal marrow becomes firmer, the muscles distinct, and a little cellular tissue is formed. The abdomen is fully covered in and the intestines are no longer visible. A little of the substance calledmeconiumeven collects in the intestines, the same as is found in at birth. It now weighs seven or eight ounces, and measures six or seven inches. The bones are ossified in a great part of their extent, and the rudiments of the second set of teeth are visible, under the first.
The uterus now is so large that it can no longer remain in the lower part of the pelvis, but is compelled to rise up into the abdomen for more room. This change of position is improperly calledquickening!Sometimes it takes place very gradually, sothat it is scarcely noticed, but more frequently it rises suddenly, disturbing all the internal organs, and causing in them considerable derangement till they accommodate themselves to the change. This occurrence often causes unnecessary alarm, though the sickness, and other unpleasant sensations, are always sufficiently annoying.
This stage corresponds with that in which the young of oviparous animals breaks the shell and escapes. The human being however, undergoes a remarkable change, and remains in the womb for a period longer than that already past, in order to become more perfected.
From four to nine months the development is proportionally much more rapid than during the first four months, owing to the circulation of perfect red blood, which is now found the same as in the adult, and is probably derived from the mother's blood vessels.
Five months.Every part is considerably increased in size, and become more perfect. The lungs enlarge, and are even capable of being, to a certain extent, dilated. The skin becomes much stronger. The situation of the nails can be discerned. The meconium is more abundant, and lower down in the intestines. The length is now eight or ten inches, and the weight fifteen or sixteen ounces.
Six months.The nails are marked. The head becomes downy, from the first development of the hair. A little fat is formed. Length twelve inches, weight from one and a half to two pounds. No indications of intellectual faculties.
Seven months.The whole being has rapidly progressed. The nails are formed, the hair is perfect, in the male the testicles descend to the scrotum, andin the female the ovaries reach the brim of the pelvis. The bones are tolerably firm, and the meconium collects in the large intestines. Length fourteen inches, weight about three pounds. Intellectual functions not yet exercised.
The two remaining months are merely devoted to further increase in size and weight. No new phenomena present themselves.
Nine months.Every function has become active. The skin becomes colored, and perspiration occurs. There are no indications of the intellectual functions, but the animal functions are remarkably active, particularly that oftaste, which no doubt leads to the act of sucking, from the natural desire for its gratification. The child can now experience all the ordinary sensations of pain, hunger, heat, and cold, and is capable of preserving an independent existence if brought into the world.
Plate XII. represents a section of the Uterus at about one month of gestation, so as to show all the parts in their proper situation.
PLATE XII.Section of the Uterus, with the Ovum and appendages, at about one month of gestation.—a. a. a.The substance of the walls of the Womb.—b. b.The Embryo.—c.The different vessels by which it is connected with the Placenta.—d. d.The Placenta.—e.The Vitellus.—f. f. f.The Membrane lining the Uterus, called the Decidua; it is seen to be bent double, or reflected, the Embryo being on the outside of it.—g. g.The Chorion, or Middle Membrane, which is studded over with villosities, or small blood vessels.—h. h.The Amnion, or inner membrane, which contains the fluid called the liquor amnii, in which the Embryo floats.—i. i.The blood vessels which connect the Placenta with the Womb.—j.Is a plug of Mucus, by which the mouth of the Womb is now blocked up.—k. k.The ends of the Fallopian Tubes, which are cut off; these are also blocked up with mucus, the same as the Os Tincæ.—l.The Os Tincæ, or mouth of the Womb.—m.The Vagina.
PLATE XII.
Section of the Uterus, with the Ovum and appendages, at about one month of gestation.—a. a. a.The substance of the walls of the Womb.—b. b.The Embryo.—c.The different vessels by which it is connected with the Placenta.—d. d.The Placenta.—e.The Vitellus.—f. f. f.The Membrane lining the Uterus, called the Decidua; it is seen to be bent double, or reflected, the Embryo being on the outside of it.—g. g.The Chorion, or Middle Membrane, which is studded over with villosities, or small blood vessels.—h. h.The Amnion, or inner membrane, which contains the fluid called the liquor amnii, in which the Embryo floats.—i. i.The blood vessels which connect the Placenta with the Womb.—j.Is a plug of Mucus, by which the mouth of the Womb is now blocked up.—k. k.The ends of the Fallopian Tubes, which are cut off; these are also blocked up with mucus, the same as the Os Tincæ.—l.The Os Tincæ, or mouth of the Womb.—m.The Vagina.
Section of the UterusPlate XII.Section of the Uterus, with the Ovum and appendages, at about one month of gestation.
Section of the Uterus
Plate XII.Section of the Uterus, with the Ovum and appendages, at about one month of gestation.
Plate XII.
Section of the Uterus, with the Ovum and appendages, at about one month of gestation.
The manner in which the new being derives its nutriment, or the material by which it grows, is, in a great measure, unknown to us, though we certainly obtain some little information about it by a study of the apparatus employed in the process.
For the first fifteen or twenty days the substance called theVitellus, (e.Plate XII.) which is analagous to the yelk of the ordinary egg, appears to supply most, if not all of the material that is required in the formation of the new being; and indeed this substance does not totally disappear till after the third month, though we cannot suppose it to be the sole source of nutriment then. It is also supposed, by some, that the amniotic liquor, in which the fœtus floats, may afford some nutriment, either by being swallowed, or by being absorbed through the skin. It is certain that this fluid is nutritive, and there is nothing impossible in its absorption, though it is not very likely to occur to a sufficient extent. The idea that it can be swallowed however, is erroneous, because the mouth of the Fœtus is firmly closed while in the Womb; and besides, children have been born alive withoutmouths, and even withoutheads, and of course they could not have swallowed anything. It is now generally conceded by physiologists that the material required by the Fœtus, for its nutrition, is obtained from the blood of the mother, through the medium of the Placenta, and the vessels in the Umbilical cord. It is, however, a matter of dispute whether the maternal blood is sent directly, in its ordinary state, into the body of the child, or whether it first undergoes a preparatory process, which most modern authors suppose it does.
From the earliest period of gestation, the middle membrane, called the chorion, (g. g.P. XII.) is covered, on its outer surface, with a number of small protuberances calledvillosities, which subsequently become true blood vessels. About the fourth month these have increased very much in size and number, and have all become conglomerated into one mass, in form like a mushroom. This is called thePlacenta. It is almost entirely formed of blood vessels, which seem to attach themselves at one end, by open mouths, to the open mouths of other blood vessels on the inner walls of the uterus (i. i.Plate XII.) At the other end these vessels are drawn together and lengthened out into a long tube, called theumbilical cord, ornavel string, which finally enters the body of the child at the navel and so establishes the connexion between it and the mother.—(c.Plate XII.)
The blood vessels in the placenta, umbilicus, and fœtus, like those in the maternal body, are of two kinds,Arteriesandveins. The arteries, which come from theleftside of the heart, carry the pure blood, which contains all the materials for forming and nourishing every part of the system. The veins contain the blood in its impure state, and take it to therightside of the heart, from whence it is forced into the lungs to be purified by the act of breathing. The blood is made impure by some of its constituents being absorbed, to form the different parts of the body, and by having thrown into it a quantity of waste and poisonous matter no longer needed.
The course of the blood, therefore, is from the left side of the mother's heart along her arteries till it reaches the arteries of the uterus, from them it passes into those of the placenta, and thence into those of the umbilicus which convey it into the body of the child.When there it circulates in its arteries, supplies the material for its further increase and development, becomes in consequence impure, and passes into its veins, the same as in the maternal body. From these veins it passes into those of the umbilicus and placenta, and, apparently, into those of the mother, by which it is conveyed to the right side of her heart, and by its action to her lungs, to be again purified when she breathes. This explains what was previously stated, that the child uses the mother's heart, lungs, and stomach, while in the womb, and has, therefore, no occasion to use its own.
The diameter of the placenta is about six inches, and its thickness about one inch and a half. The length of the umbilical cord is from eighteen to twenty-four inches, its diameter about half an inch. These dimensions are, however, subject to great variation. Instances are mentioned of the cord being five feet long, and as thick as the child's arm. I have seen one myself four feet long. Sometimes it will be very short, not more than eight or ten inches. It is composed of one artery and two veins, twisted together like the strands of a cable, and of a sheath surrounding them composed of the chorion and amnion. Between the sheath and the vessels is a thick gelatinous fluid called the Gelatine of Wharton.
This explanation, it must be remembered, is in fact merely hypothetical. The direct passage of the blood through the Placenta, from the mother's vessels into those of the cord, is denied by many physiologists, who contend that there is an intermediate set of vessels in the Placenta, in which it first undergoes important changes. They also contend that the impure blood does not pass through into the mother's veins at all, but is purified in the Placenta,and immediately returned. Some have even averred that the Placenta is not required at all, to supply nourishment, but is merely a purifying organ. It is now known, however, that it is not absolutely essential to either process, for children have been born alive, and perfectly formed, which merely floated loosely in the amniotic liquor, having neither Placenta nor cord, nor any other connection with the mother. How they were nourished we cannot tell. These, however, must be regarded merely as curious exceptions, there being little doubt but that fœtal nutrition is ordinarily effected through the Placenta and cord, by means of the mother's blood, somewhat in the manner we have described.
From the circumstance of the fœtus not using its heart and lungs, like the adult, its circulation has several modifications.
The engine by which the blood is forced along its vessels is theheart!This is divided into two distinct parts, each of which has two cavities, the upper one called theauricle, and the lower one theventricle, which communicate with each other by curious valves. In the adult the whole of the impure blood is poured into the right auricle, that from the lower part of the body by theinferior vena cava, and that from the upper part by thesuperior vena cava. From the right auricle it passes into the right ventricle, which pumps it into the lungs, by way of the pulmonary artery; here it is purified by the act of respiration, and then brought, when pure, by the pulmonary veins, into the left auricle, and passes fromthence into the left ventricle, which pumps it into the great aorta, and from thence into the smaller arteries all over the body. The two sides of the heart, therefore, do not communicate directly with each other, but there is a strong partition between them. In the fœtus the arterial blood from the mother, when it leaves the umbilical artery, enters first the liver, runs through its vessels, gives off the bile found in it, and then joins the vena cava inferior. By this passage it is taken into the right auricle, along with the impure blood of the vena cava. From the right auricle it passes through a hole in the partition directly into the left auricle, instead of taking the indirect route by the lungs as in the adult. From the left auricle it passes into the left ventricle, and is from thence distributed by the arteries all over the body. This opening in the partition is called theforamen ovale!
After birth, when the blood begins to pass through the lungs, this passage closes up. By the eighth day it is generally obliterated, often much sooner, though occasionally it has remained open longer without inconvenience. In some cases the foramen ovale does not close at all. The child then has what is called theblue disease!The whole body is of a uniform leaden, or blue color, and the whole system is generally languid and sluggish. The blue color is caused by the dark blood of the veins mixing with that of the arteries. These children mostly die early, but some live to be five or six years old, and one I saw twelve, but this is rare. No remedy can be had for this affliction, and I have never known it to cure spontaneously. Some children are so very dark for a few days after birth as to cause great alarm. This is owing to the foramen ovale being very open andclosing slowly. No apprehension need be experienced in such cases, as it soon subsides.
The impure blood from the upper part of the fœtal body, which is brought down by the superior vena cava, also enters the right auricle, but does not pass from thence through the foramen, like that from the inferior vena cava. By a peculiar arrangement this blood is made to pass down into the right ventricle, and from thence along the pulmonary artery, the same as in the adult state. Only a very small portion, however, passes into the lungs, the great part being taken along a tube called theductus arteriosusinto the great artery called the aorta, where it begins to turn down to the lower part of the body. In consequence of this, the arterial blood going down to the lower part of the body, is mixed with this portion of impure, venous blood, brought by the ductus arteriosus from the superior vena cava; while that going to the head, and upper part of the body remains pure. And this is the reason why the lower part is always so much smaller than the upper part, previous to birth; it receives less pure nourishment. The head and chest appear, at an early period, almost as large as the rest of the body.
This circumstance also explains why, in the great majority of cases, therightarm is preferred to theleft, and has more real power. The place where the ductus arteriosus pours the impure blood into the aorta, is almost immediately opposite to where the artery is given off which feeds the left arm. In consequence of which, in most cases, a small portion of this impure blood becomes mixed with the arterial blood, and the left arm is, therefore, in the same situation as the lower limbs, and like them is comparatively imperfectly developed. The right arm isnot liable to any such deprivation. In some cases the insertion of the ductus arteriosus is lower down, so that no such mixture occurs. Both arms are then equal, and this accounts for the fact that in some persons there appears to be no difference. In some cases, no doubt, early habit, or imitation, may overcome this natural inferiority, and even give the preference to the left arm; but such instances are rare; the general rule is the contrary, and for the reason stated.
The ductus arteriosus closes up about the same time as the foramen ovale.
The two veins which convey the impure blood back to the mother, to be purified, originate from the iliac artery, in the pelvis. They pass up the sides of the bladder towards the navel, enter the sheath of the cord, and so reach the placenta. These vessels are obliterated about the third or fourth day after birth, and remain afterwards in the form of a fibrous cord.
The real source ofallthe blood in the body of the child is a mystery; it would certainly appear most likely for the whole of it to be derived from the mother's vessels, but there are many circumstances which make it probable that the child may form some itself, by digesting the fluid it is supposed to absorb. This view is supported by the fact that there is found in its bowels at birth, and even before, a greenish substance like excrement, calledMeconium. This has every appearance of being the product of digestion, though some suppose it to be derived from the liver. It occasionally contains hair, and other anomalous substances.