FOETUS (Lat., offspring). The term applied to the unborn young of viviparous animals, especially in the more advanced stages. in the human subject the tern fictus is applied to the embryo. though more often to the unborn child after the end of the fourth month, when bunion features are distinguishable. The fourth month ffetns weighs about five ounces, and is about six inches long: its brain convolutions are found to lie developing, and its sex is recognizable; its muscles are capable of pro ducing movements of the limbs, and its extend ing ossification can lie traced in the bones of the skull. The fifth-month het us weighs about ten ounces, and is about ten inches long; hair and nails appear, and ossification in the ischium takes place in course of the fifth month. The sixti A-month f(eLUS weighs about one pound, and is eleven or twelve inches long; its eyelhls, with eyelashes formed, are dosed; fat gradually develops under its skin, and its pubic bones undergo ossification. The seventh.monib f•tus weighs front three to four pounds, and is front thirteen to fifteen inches long; its skin is env erect with sebaceous matter, and the fat under its skin attains to a considerable amount ; its eyelids are open. The eighth-month foetus weighs from four to five pounds, and is front sixteen to eighteen inches long; its nails are completely developed, and the membrana pupillaris has dis appeared. The ninth-month, or full term, f•tus weighs front five to nine pounds, or six and one half pounds on an average, and is from seventeen to twenty-one inches long. 'Three average weight and length of the male infant slightly exceed those of the female, even at birth; the average excess in weight is about ounces, the aver age excess in length about 4.8 inches. Clark observes that if, at the full term, the weight of the infant is less than five pounds it rarely thrives, though a few cases are recorded of sur viving children who weighed one pound at birth. On the other hand, various instances are recorded of infants in which the weight at birth exceeded twice the average weight. There :ire certain points in which the fetus at the full period dif fers anatomically from the child shortly. after birth. The bony skeleton is very incomplete, car tilage occurring in the place of many bones. In deed, complete ossification (e.g. of the vertebra.) is not finished until about the twenty-fifth year, and the only bones completely ossified at birth are the minute ossicles of the ear. The differ ence between the foetus and the child in this re spect is, however, only one of degree.
During pregnancy. a temporary organ, termed the placenta (popularly known as the afterbirth, from its being delivered shortly after 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 um bilical cord, in which lie the umbilical vein, which conveys arterial blood to the fetus, and the two umbilical arteries, which return the blood to the placenta. This umbilical cord conveys these vessels to the umbilicus, or navel. In
the heart we find a communication between the two auricles by means of an opening termed the foramen orale. In the arterial system we have to notice, first, the duct us artcriosus, which is a large communicating trunk between the pul monary artery and the descending aorta; and, secondly, the branches given off by the internal iliac arteries, which go under the name of hypo gastric as long as they are within the body of the fcelus, and of umbilical when they enter into the structure of the cord, and are continued from the foetus to the placenta, to which they return the blood which has circulated in the fetal system. In the venous system there is a communication between the umbilical vein and the inferior veim cavil, called the ductus venoms.
Pure blood is brought front the placenta by the umbilical vein. which passes through the um bilicus, and enters the liver. where it divides into several branches, which are distributed to that viscus, the main trunk, or dnetus renosus, pass ing directly backward, and entering the inferior vena eava. The pure blood here becomes mixed ith the impure blood Much is ran from the lower extremities and abdominal viscera, and I, l'a Friel] into t he right auricle, and troy' thence, guided by the Eustachian vatic (which is situ• ated betw.cen the anterior margin of the inferior eava and the atiricubeventricular orifice, :End is of relatively large size in the fo•usi, passes I }trough the foram, a orate, into the left ;lurid, Drum the left auricle it passes into the 1141 ien trifle, and into the aorta, whence it is dis• tributed by the carotid and subelavian arteries principally to the head :Ind tipper extremities, h•li reccivc enitipa rat i Velly pure blood. From the head and arms the impure blood is re1111111•11 by the ,111,4•11'1,1* lava lu 1.11.• 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 foetus it passes through the duct as arteriosus into the elonmencem•nt of the descending aorta, where it mixes with that portion of the pure blood which is not sent through the carotid and subehivian arteries. Some of this mixed blood is distributed by the external iliac arteries to time lower ex t remit ieS. While t he remainder ( probably t he larger portion) is conveyed by the hypogastrie or umbilical arteries to the placenta. Almost im mediately after birth, the foramen orate becomes closed by a membranous layer, and the dnetus nrtrriosus and du•tns renosus rapidly •hose, and degenerate into impervious fibrous cords. The lungs, previously to the act of inspiration. are dense and solid in and of a deep-red color, and lie fat' back in the chest. Their spe cific gravity is greater than water. in which they tor portions of them) consequently sink, whereat or portions of lungs, that have respired. tloat in that fluid.