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.
PECULIARITIES OF THE FŒTAL CIRCULATION.
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 the heart! This is divided into two distinct parts, each of which has two cavities, the upper one called the auricle, and the lower one the ventricle, 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 the inferior vena cava, and that from the upper part by the superior 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 from thence 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 the foramen 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 the blue 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 and closing 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 the ductus arteriosus into 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, the right arm is preferred to the left, 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 is not 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 of all the 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, called Meconium. 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.
CHAPTER IV.
THE BREAST.
The Breasts, or Mammæ, are not needed in the process of generation, nor are they absolutely necessary even after birth; but as they are naturally associated, in the majority of cases, with infantile nutrition, and are besides liable to many derangements and diseases during pregnancy and child-birth, it is advisable to give some account of them.
When one of the breasts is dissected it is found to be composed chiefly of a singular body called the Mammary Gland, which resembles somewhat a very firm piece of fat, of a yellowish drab color. In the substance of this Gland are an immense number of little cells, or vessels, in which, by some unexplainable process, the milk is secreted, or made from the blood. From these little vessels there proceeds small tubes which gradually unite into larger ones, and these again into larger ones still, until at last all the milk is poured into a few tubes, or canals, which terminate in the nipple. The outer mouths of these terminal canals are only slightly contracted together, so that the suction of the child's mouth, or even the pressure of the milk, when the breast is full, will force them open and allow the fluid to flow out.
The following plate represents the structure of the Breast, and explains, as far as it can be explained, the manner in which it performs its functions.