FCE'TUS, the term applied in medicine to the mammalian embryo, especially in its more advanced stages. In the human subject, we usually speak of the embryo at and after the end of the fourth month as a foetus.
There are several points in relation to the F. which are of great interest both to the physiologist and to the medical jurist. It is frequently of great importance in medico legal inquiries to be able to ascertain the age of the F.; and tofacilitate such determina tion, the physical characters which it presents at different ages have been carefully noted and described.
In the F. of nine months—the full term—the length is from 17 to 21 in.; weight from 5 to 9 lbs., the average being about lbs. Even at birth, the average length and weight of the male infant slightly exceeds that of the female. From numerous obser vations made by Quetelet, it appears that there is an average excess of length of 4.8 lines, and of weight of 12 ounces, in the male infant.
The average weight of infants, without regard to sex, was found by a French observer, Chaussier (who noted the weight in more than 20,000 cases), to be about 6.7 lbs.—the maximum being 11.3, and the minimum 3.2 lbs. From the inquiries of Dr. Joseph Clark (Philosophical lransactiom, vol. 76), which were made on 60 males and 60 females, the average in this country seems rather higher, the weight of males being 7 lbs. 6 oz., and that of the feniales being nearly 6 lbs. ‘12 oz. ' • and sir James Y. Simpson arrived at very nearly the same result. Clark observes that if, at the full time, the weight of the infant is less than 5 lbs., it rarely thrives. Various instances are recorded of infants in which the weight at birth has exceeded twice the average weight. Thus a case is recorded by Mr. Owen, in the Lancet for 1838, in which the child at delivery weighed 17 lbs. LI oz., and was 24 in. in length; and in the Mcdico-Chir. Review, Oct., 1841, there is the mention of a case in which the weight was nearly 18 lbs.
There are certain points in which the F. at the full period differs anatomically from the child shortly after birth. The bony skeleton is very incomplete. cartilage occurring in the place of many bones. Indeed, complete ossification (viz., of the vertebrae) is not finished until about the 25th year, and the only bones completely ossified at birth arc the minute ossicles of the ear. The difference between the F. and the child in this respect is, however, only one of degree. h,, /n) During pregnancy, a temporary organ, termed the placenta (popularly known as the afterbirth, from its being thrown off shortly after the birth of the child), is developed on the inner wall of the uterus, This organ is mainly composed of vessels, and there proceeds from it the structure known as the umbilical cord, in which lie the umbilical vein, which conveys arterial blood to the F., and the two umbilical arteries, which
return the blood to the placenta This umbilical cord conveys these vessels to the umbilicus, or navel. Before tracing the course of the blood through the F., we must notice the chief anatomical peculiarities presented by the vascular or circulating system before birth.
1. In the heart, we find a communication between the two auricles by means of an opening termed the foramen ovale. 2. In the arterial system, we have to notice first, the ductus arteriosus, which is a large communicating trunk between the pulmonary artery and the descending aorta; and, secondly, the branches given off by tlke internal iliac arteries, which go under the name of hypogastric as long as they are within the body of the. F., and of umbilical when they enter into the structure of the cord, are continued from the F. to the placenta, to which they return the blood which has circu lated in the festal system. 3. In the venous system there is a communication between the umbilical vein and the inferior vena cava, called the ductus venosus.
Pure blood is brought from the placenta by the umbilical vein, which passes through the umbilicus, and enters the liver, where it divides into several branches, which are distributed to that viscus, the main trunk or ductus venosus, passing directly backwards, and entering, the interior vena cava. The pure blood here becomes mixed with the Impure blood which is returned from the lower extremities and abdomipial viscera, and is carried into the .right auricle, and from thence, guided by the eustachian valve (which is situated between the anterior margin of the inferior cava, and the aurieulo ventricular orifice, and is of relatively large size in the F.), passes through the fora men oval, into the left auricle. From the left auricle, it passes into the left ventricle, and into the aorta, whence it' is distributed by the carotid and subclavian arteries principally to the head and upper extremities, which thus receive comparatively pure blond. From the head and arms, the impure blood is returned by the superior versa cava to the right auricle; from the right auricle, it is propelled, as in the adult, into the right ventricle; and from the right ventricle, into the pulmonary artery. In the adult, it would now pass through the lungs, and be oxygenized; but in the F., it passes through the ductus arteriosus into the commencement of the descending aorta, where it mixes with that portion of the pure blood which is not sent through the carotid and subclavian arteries. Some of this mixed blood is distributed by the exter nal iliac arteries to the lower extremities, while the remainder (probably the larger portion) is conveyed by the hypogastric or umbilical arteries to the placenta.