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fætus be born at the end of five months, it may live for a few minutes.
In the sixth month the dermis begins to be distinguished from the epidermis. The skin is delicate, smooth, and of a purple colour; especially on the face, lips, ears, palms of the hands and soles of the feet. It seems plaited, owing to the absence of fat in the subcutaneous cellular tissue. The scrotum is small, and of a vivid red hue. The vulva is prominent, and its lips are separated by the projection of the clitoris. The nails appear, and, towards the termination of the month, are somewhat solid. Should the fœtus be born now, it is sufficiently developed to breathe and cry, but it dies in a few hours. Length, at six months, ten or twelve inches. Weight under two pounds.
During the seventh month, all the parts are better proportioned. The head is directed towards the orifice of the uterus, and can be felt by the finger introduced into the vagina, but it is still very movable. The eyelids begin to separate, and the membrane, which previously closed the pupil—the membrana pupillaris—to disappear. The fat is more abundant, so that the form is more rotund. The skin is redder, and its sebaceous follicles are formed, which secrete a white, sebaceous substance that covers it; and the testicles are in progress to the scrotum. The length at seven months is fourteen inches; the weight under three pounds.
In the eighth month, the foetus increases proportionably more in breadth than in length. All its parts are firmer and more formed. The nails exist; and the child is now certainly viable, or capable of supporting an independent existence. The testicles descend into the scrotum; the bones of the skull, ribs, and limbs are more or less completely ossified. The length is sixteen inches; the weight four pounds and upwards.
At the full period of nine months, the organs have acquired the development necessary for the continued existence of the infant. Length eighteen or twenty inches; weight six or seven pounds. Dr. Dewees says the result of his experience, in this country, makes the average weight above seven pounds.
The whole of this description amounts to no more than an approximation to the truth. The facts will be found to vary greatly in individual cases, and according to individual experience: and this accounts for the great discordance in the statements of different observers. This discordance is strongly exemplified in the following table, containing the estimates of the length and weight of the foetus at different periods of intra-uterine existence; as deduced by Dr. Beck from various observers, and as given by Maygrier on his own authority. It is proper to remark, that the Paris pound, poids de marc, of sixteen ounces, contains 9216 Paris grains, whilst the avoirdupois contains only 8532.5 Paris grains; and that the English inch is 1.065977 Paris inch.
At 30 days, 13 to 5 lines 10 to 12 lines
19 or 10 grains 2 months, 2 inches 4 inches 2 ounces 5 drachms 3 do.
3} do. 6 do. 2 to 3 ounces 21 ounces 4 do. 5 to 6 do. 8 do. 4 or 5 do. 7 or 8 do. 5 do. 7 to 9 10 do.
9 or 10 16 ounces 6 do. 9 to 12 12 do.
1 to 2 pounds 2 pounds 7 do. 12 to 14 14 do. 2 to 3 do. 3 do. 8 do. 16 16 do. 3 to 4 do. 4 do.
The difficulty must necessarily be great in making any accurate estimate during the early periods of foetal existence; and the changes in the after months are liable to considerable fluctuation. Chaussier affirms, that after the fifth month, the fœtus increases an inch every fifteen days, and Maygrier adopts his estimate. The former gentleman has published a table of the dimensions of the fœtus at the full period, deduced from an examination of more than fifteen thousand cases. From these we are aided in forming a judgment of the probable age of a fœtus in the latter months of utero-gestation;—a point of interest with the medico-legal inquirer. At the full period, the middle of the body corresponds exactly with the umbilicus; at eighth months, it is three-quarters of an inch, or an inch higher. At seven months it approaches still nearer the sternum; and at six months it falls exactly at the lower extremity of that bone; hence, if we depend upon these admeasurements, should the middle of the body of the fœtus be found to fall at the lower extremity of the sternum, we may be justified in concluding that the foetus is under the seventh month, and consequently not viable or rearable. A striking circumstance, connected with the development of the foetus, is the progressive diminution in the proportion between the part of the body above the umbilicus and that below it. At a very early period of fœtal life, (see Figs. 152, and 153,) the cord is attached near the base of the trunk; but the parts beneath become gradually developed, until its insertion ultimately falls about the middle of the body.
The position of the fœtus in utero, and the cause of such position 'at various periods of utero-gestation, have been topics of some interest. In the early weeks, it seems to be merely suspended by the cord; and it has been conceived, that because the head is heavier, it is the lowest part. It is difficult, however, to admit this as the cause of the position assumed in such an immense majority of cases, or to fancy, that the nice adaptation of the fœtal position to the parts through which the child has to pass is simply dependent upon such
a mechanical cause. Gravity can afford us no explanation why the face, in 12,120 cases out of 12,633, has been found turned to the right sacro-iliac synchondrosis, (see Fig. 144,) and the occiput to the left acetabulum; and in the 63 of these cases in which the face was turned forward, and in the 198 breech presentations, are we to presume, that the whole effect was owing to mere difference of weight in those parts that were lowest?
The common position of the fœtus, at the full period, is exhibited in the above illustration. The body is bent forward, the chin resting on the chest, the occiput towards the brim of the pelvis, the arms approximated in front, and one or both lying upon the face; the thighs bent upon the abdomen, the knees separated, the legs crossed, and drawn up, and the feet bent upon the anterior surface of the leg; so that the oval, which it thus forms, has been estimated
at about ten inches in the long diameter; the head, at the full period, resting on the neck, and even on the mouth, of the womb, and the breech corresponding to the fundus of the organ.
From the first moment of a fecundating copulation, the minute matters, furnished by both sexes, when commingled, commence the work of forming the embryo. For a short time, they find in the ovum the necessary nutriment, and subsequently obtain it from the uterus. The mode, in which this action of formation is accomplished, is as mysterious as the essence of generation itself. When the impregnated ovum is first seen, it seems to be an amorphous, gelatinous mass, in which no distinct organs are perceptible. In a short time, however, the brain and spinal marrow, and blood-vessels, make their appearance, but which of these is first developed is undecided.
Sir EVERARD IIOME,—from his observations of the chick in ovo, as well as from the microscopic appearances, presented by the ovum in the case of the female who died on the seventh or eighth day after impregnation, in which a rudimental brain and spinal marrow were perceptible,—decides, that the parts, first formed, bear a resemblance to brain, and that the heart and arteries are produced in consequence of the brain having been established. Rolando also assigns the priority to the nervous system. Meckel, however, admits no primitive organizing element, but believes, that the first rudiments of the fœtus contain the basis of every part. On the other hand, the recent researches of Serres, on the mode of development of the nervous system, have induced him to be in favour of the earlier appearance of the blood-vessels; and this view appears to be supported by the fact, that if an artery of the brain is wanting or is double, the nervous part to which it is usually distributed is also wanting or double. The acephalous foetus has no carotid or vertebral arteries; whilst the bicephalous or tricephalous have those vessels double or treble.
The successive development of the different organs is a topic of deep interest to the student of general anatomy, and has engaged the attention of some of the most excellent anatomists of modern times. Amongst these, Rolando, Tiedemann, Ackermann, Serres, Velpeau, Walther, Beclard, Rosenmuller, and Meckel, are especially conspicuous. The nature and limits of this work will preclude us from entering into this investigation any farther than to point out some of the most striking peculiarities of foetal life.
The head of the fœtus is large in proportion to the rest of the body, and the bones of the skull are united by membrane; the sagittal suture extends down to the nose, so as to divide the frontal bone into two portions; and where this suture unites with the coronal, a quadrangular space is left, filled up by membrane, which is called the anterior fontanelle or bregma. Where the posterior extremity of the sagittal suture joins the lambdoidal, a triangular
space of a similar kind is left, called the posterior fontanelle or posterior bregma. It is important for the obstetrical practitioner to bear in mind the shape of these spaces, as they indicate to him whether the anterior or posterior part of the head is the presenting part.
The pupil of the eye, in a very young fœtus, is entirely closed by a membrane, called membrana pupillaris, which arises from the inner margin of the iris, and continues there till the seventh month, when it gradually vanishes by absorption. It is a vascular substance, and, like the iris, to which it is attached, separates the two chambers of the eye from each other. Wachendorff first described it in 1738; and both he and Wrisberg detected vessels in it. Its vascularity was denied by Bichat, but it has been satisfactorily demonstrated by J. Cloquet. The membrane is manifestly connected with the process of formation of the delicate organ to which it is attached; and, according to Blumenbach, it keeps the iris expanded, during the rapid increase of the eyeball.
In the upper part of the thorax of the fœtus, a large gland or rather glandiform ganglion exists, called the thymus. It is situated in the
Fig. 156. superior mediastinum, and lies over the top of the pericardium and the arch of the aorta. It has two long cornua above, and two broad lobes below. Its appearance is glandular, and colour very variable. In the progress of age it diminishes, so that in the adult it is wasted, and in old age can scarcely be discovered amongst the cellular tissue. It is surrounded by a thin, cellular capsule, which sends prolongations into its interior, and divides it into lobules of unequal size, on which several vesicles are distinguishable, filled with a milky fluid.
The thymic arteries proceed from the inferior thyroid, internal mammary, bronchic, mediastinal, &c. The nerves proceed from the pneumogastric, diaphragmatic, and inferior cervical ganglia. It has no excretory duct; and is one of the most a,.. Divided integuments.-c, c. Divided obscure, in its physiology, of any of thymus gland.-8,8, ,!. Lungs.-. Right the organs of the body.
auricle of the heart.- k. Right ventricle.
n, o. Right and left lobes of the liver.-P. StoThe thyroid gland, which has been macheni ( Small intestines. The colon. described in another place, and whose chusi raz " The umbilical arteries.-The functions are equally obscure, is also collapsed umbilical cord. largely developed in the fœtus; as well as the supra-renal capsules.