LETTER XXIII.. Anatomy of the Fetus.

ANATOMY OF THE FETUS.

Its Length and Weight—It Osseous, Muscular, and Vascular Systems—The Fetal Circulation—Nervous System—Organs of Sense—Its Lungs, Heart, and other Internal Organs.

As a matter of natural science, rather than one of real practical utility, to you for whom I write these letters, I present you with a description of some of the more important characteristics of the fetus. In doing this I acknowledge myself indebted to Mr. Wilson, the distinguished anatomist before referred to.

The medium weight of a child of the full period, at birth, is seven pounds, and its length seventeen inches; the extremes of weight are four pounds and three quarters, and ten pounds; and the extremes of measurement fifteen and twenty inches. The head is of large size, and lengthened from before backward; the face small. The upper extremities are greatly developed, and the thorax expanded and full. The upper part of the abdomen is large, from the great size of the liver; the lower part is small and conical; and the lower extremities are very small in proportion to the rest of the body. The external genital organs are very large, and fully developed, and the attachment of the umbilicus is one inch farther from the vertex of the head than from the soles of the feet; and one inch farther from the ensiform cartilage than from the symphysis pubis.

OSSEOUS SYSTEM.

The development of the osseous system is treated of in the various works on anatomy. The ligamentous system presents no peculiarity deserving of remark.

MUSCULAR SYSTEM.

The muscles of the fetus at birth are large and fully formed; they are of a lighter color than those of the adult, and of softer texture. The transverse striæ on the fibers of animal life are not distinguishable until the sixth month of fetal life.

VASCULAR SYSTEM.

The circulating system presents several peculiarities: 1st. In the heart; there is a communication between the two auricles by means of the foramen ovale. 2d. In the arterial system; there is a communication between the pulmonary artery and descending aorta, by means of a large trunk, the ductus arteriosus. 3d. Also in the arterial system; the internal iliac arteries, under the name of hypogastric and umbilical, are continued from the fetus to the placenta, to which they return the blood which has circulated in the system of the fetus. 4th. In the venous system; there is a communication between the umbilical vein and the inferior vena cava, called the ductus venosus.

FETAL CIRCULATION.

The pure blood is brought from the placenta by the umbilical vein. The umbilical vein passes through the umbilicus, and enters the liver, where it divides into several branches, which may be arranged under three heads: 1st. Two or three, which are distributed to the left lobe. 2d. A single branch, which communicates with the portal vein in the transverse fissure, and supplies the right lobe. 3d. A large branch, the ductus venosus, which passes directly backward, and joins the inferior cava. In the inferior cava the pure blood becomes mixed with that which is returning from the lower extremities and abdominal viscera, and is carried through the right auricle (guided by the Eustachian valve), and through the foramen ovale, into the left auricle. From the left auricle it passes into the left ventricle, and from the left ventricle into the aorta, whence it is distributed, by means of the carotid and subclavian arteries, principally to the head and upper extremities. From the head and upper extremities, the impure blood is returned by the superior vena cava to the right auricle; from the right auricle, it is propelled into the right ventricle; and from the right ventricle into the pulmonary artery. In the adult, the blood would now be circulated through the lungs, and oxygenated; but in the fetus the lungs are solid, and almost impervious. Only a small quantity of the blood passes therefore into the lungs; the greater part rushes through the ductus arteriosus, into the commencement of the descending aorta, where it becomes mingled with that portion of the pure blood which is not sent through the carotid and subclavian arteries.

Passing along the aorta, a small quantity of this mixed blood is distributed by the external iliac arteries to the lower extremities; the greater portion is conveyed by the internal iliac, hypogastric, and umbilical arteries to the placenta; the hypogastric arteries proceeding from the internal iliacs, and passing by the side of the fundus of the bladder, and upward along the anterior wall of the abdomen to the umbilicus, where they become the umbilical arteries.

FETAL CIRCULATION.

1. The umbilical cord, consisting of the umbilical vein and two umbilical arteries; proceeding from the placenta (2). 3. The umbilical vein dividing into three branches; two (4, 4), to be distributed to the liver; and one (5), the ductus venosus, which enters the inferior vena cava (6). 7. The portal vein, returning the blood from the intestines, and uniting with the right hepatic branch. 8. The right auricle; the course of the blood is denoted by the arrow, proceeding from 8 to 9, the left auricle. 10. The left ventricle; the blood following the arrow to the arch of the aorta (11), to be distributed through the branches given off by the arch to the head and upper extremities. The arrows 12 and 13, represent the return of the blood from the head and upper extremities through the jugular and subclavian veins, to the superior vena cava (14), to the right auricle (8), and in the course of the arrow through the right ventricle (15), to the pulmonary artery (16). 17. The ductus arteriosus, which appears to be a proper continuation of the pulmonary artery, the offsets at each side are the right and left pulmonary artery cut off; these are of extremely small size as compared with the ductus arteriosus. The ductus arteriosus joins the descending aorta (18, 18), which divides into the common iliacs, and these into the internal iliacs, which become the hypogastric arteries (19), and return the blood along the umbilical cord to the placenta; while the other divisions, the external iliacs (20), are continued into the lower extremities. The arrows at the terminations of these vessels mark the return of the venous blood by the veins to the inferior cava. ]

NERVOUS SYSTEM.

The brain is very soft, almost pulpy, and has a reddish tint throughout; its weight at birth, relatively to the entire body, is as 1 to 6, and the difference between the white and gray substance is not well marked. The nerves are firm and well developed.

ORGANS OF SENSE.

Eye.—The eyeballs are of large size, and well developed at birth. The pupil is closed by a vascular membrane called the membrana pupillaris, which disappears at about the seventh month. Sometimes it remains permanently, and produces blindness. It consists of two thin membranous layers, between which the ciliary arteries are prolonged from the edge of the iris, and form arches and loops by returning to it again, without anastomosing with those of the opposite side.

The removal of the membrane takes place by the contraction of these arches and loops toward the edge of the pupil. The capsule of the lens is extremely vascular.

Ear.—The ear is remarkable for its early development; the labyrinth and ossicula auditus are ossified at an early period, and the latter are completely formed before birth. The only parts remaining incomplete are the mastoid cells, and the meatus auditorius. The membrana tympani in the fetal head is very oblique, occupying almost the basilar surface of the skull; hence, probably, arises a deficient acuteness in the perception of sound. It is also extremely vascular.

Nose.—The sense of smell is imperfect in the infant, as may be inferred from the small capacity of the nasal fossæ, and the non-development of the ethmoid, sphenoid, frontal, and maxillary sinuses.

FETAL LUNGS.

The lungs, previously to the act of inspiration, are dense and solid in structure, and of a deep-red color. Their specific gravity is greater than water, in which they sink to the bottom; whereas lung which has respired will float upon that fluid. The specific gravity is, however, no test of the real weight of the lung, the respired lung being actually heavier than the fetal. Thus the weight of the fetal lung, at about the middle period of uterine life, is to the weight of the body as 1 to 60. But, after respiration, the relative weight of the lung to the entire body is as 1 to 30.

FETAL HEART.

The heart of the fetus is large in proportion to the size of the body; it is also developed very early, representing at first a simple vessel, and undergoing various degrees of complication until it arrives at the compound character which it presents after birth. The two ventricles form, at one period, a single cavity, which is afterward divided into two by the septum ventriculorum. The two auricles communicate up to the moment of birth, the septum being incomplete, and leaving a large opening between them, the foramen ovale (foramen of Botal).

The ductus arteriosus is another peculiarity of the fetus connected with the heart; it is a communication between the pulmonary artery and the aorta. It degenerates into a fibrous cord after birth, from the double cause of a diversion in the current of the blood toward the lungs, and from the pressure of the left bronchus, caused by its distention with air.

VISCERA OF THE ABDOMEN.

At an early period of uterine life, and sometimes at the period of birth, as Dr. Wilson observed in the imperfectly-developed fetus, two minute fibrous threads may be seen passing from the umbilicus to the mesentery. These are the remains of the omphalo-mesenteric vessels.

The omphalo-mesenteric are the first-developed vessels of the germ; they ramify upon the vesicula umbilicalis, or yolk-bag, and supply the newly-formed alimentary canal of the embryo. From them, as from a center, the general circulating system is produced. After the establishment of the placental circulation they cease to carry blood, and dwindle to the size of mere threads, which may be easily demonstrated in the early periods of uterine life; but are completely removed, excepting under peculiar circumstances, at a later period.

The stomach is of small size, and the great extremity but little developed. It is also more vertical in direction the earlier it is examined, a position that would seem due to the enormous magnitude of the liver, and particularly of its left lobe.

The appendix vermiformis cæci is long and of large size, and is continued directly from the central part of the cul-de-sac of the cæcum, of which it appears to be a constricted continuation. This is the character of the appendix cæci in the higher quadrumana.

The large intestines are filled with a dark-green, viscous secretion, called meconium, from its resemblance to the inspissated juice of the poppy.

The pancreas is comparatively larger in the fetus than in the adult.

The spleen is comparatively smaller in the fetus than in the adult.

FETAL LIVER.

The liver is the first-formed organ in the embryo. It is developed from the alimentary canal, and at about the third week fills the whole abdomen, and is one half the weight of the entire embryo. At the fourth month the liver is of immense size in proportion to the bulk of the fetus. At birth it is of very large size, and occupies the whole upper part of the abdomen. The left lobe is as large as the right, and the falciform ligament corresponds with the middle line of the body. The liver diminishes rapidly after birth, probably from obliteration of the umbilical vein.

KIDNEYS AND SUPRA-RENAL CAPSULES.

The kidneys present a lobulated appearance in the fetus, which is their permanent type among some animals, as the bear, the otter, and cetacea.

The supra-renal capsules are organs which appear, from their early and considerable development, to belong especially to the economy of the fetus. They are distinctly formed at the second month of embryonic life, and are greater in size and weight than the kidneys. At the third or fourth month, they are equaled in bulk by the kidneys; and at birth, they are about one third less than those organs.

VISCERA OF THE PELVIS.

The bladder in the fetus is long and conical, and is situated altogether above the upper border of the ossa pubis, which are as yet small and undeveloped. It is, indeed, an abdominal viscus, and is connected superiorly with a fibrous cord, called the urachus, of which it appears to be an expansion.

The urachus is continued upward to the umbilicus, and becomes connected with the umbilical cord. In animals it is a pervious duct, and is continuous with one of the membranes of the embryo, the allantois. It has been found pervious in the human fetus, and the urine has been passed through the umbilicus. Calculous concretions have also been found in its course.

The uterus, in the early periods of embryonic existence, appears bifid, from the large size of the Fallopian tubes, and the small development of the body of the organ. At the end of the fourth month, the body assumes a larger bulk, and the bifid appearance is lost. The cervix uteri in the fetus is larger than the body of the organ.