F<ETUS. 7 hair and nails appear, and ossification in the ischium takes place in com n ol i be nn h mi The sixth-month foetus weighs al me pound, and is eleven or i welve inches loi with eyela ihes formed, a develops under its skin, and its pubic bi undergo ossification. The evei ih n weighs from three to four pounds, and is thirteen I" fifteen inches Ion in i- coa ered with sebaceous mat tei and th Eal under its skin attains to a considerable ai mi : it 3 eyelids are open. The eighth month foeti four to five pounds, and is from sixteen to eighteen inches long; it- nails are compli developed, and the membrana pupillaris ha appeared. The ninth month, or full era tel us weighs from five to nine pound half 1 ids on an average, and is from seventeen to twentj one inches long. The average weight and length of the male infant slightly e those "i the female, run al birth; thi excess in weight is about twelve ounces the 1 age excess in length about I 8 im I lark observes that if, at the full term, the weigh! the infanl is less than live pounds h rarely thrives, though a few cases are recorded of sur- viving children who weighed "in' pound al birth. On the other hand, various instances are recorded of infants in which the weight at birth twice the average weight. There are certain points in which the foetus 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 vertel is not finished until about the twenty fifth year, and the only bones completely ossified at birth arc 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 chi] 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 inn bilical cord, in which lie the umbilical vein, which conveys arterial blood to the foetus, 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 ovale. In the arterial system we have to notice, first, the ductus arteriosus, which is a large communicating trunk between the pul monary artery and the descending aorta; and. secondly, the branches given nil' by the internal iliac arteries, which go under the name of hypo- gastric as long as they are within the body of (lie foetus, 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 foetal system. In the venous system there is a communication between the umbilical vein and the inferior ven 1 cava, called the ductus venosus. Pure blood is brought from the placenta by the umbilical vein, which passe- through the um- bilicus, and enters the liver, where if divides into several branches, which arc distributed f" tli.it viseus, the main trunk, or ductus venosus, pass ing directly backward, and entering the inferior 1 ■ > FOG. 1. The pure id 1 here becomi with the impure blood whi iirned from 1 and is carried into 1 1 Eustachian valve (whicn is »itu ated bet ween 1 1 1 margin ■>> 1 be infi 1 ioi a 1 ad ■ auriculo vent 1 icutai 1 ad i^ of n da) ively It in the feel us 1 . pa From 1 he left am icli it pa ■ i left ven tricle, and into the aorta, when dis ited by the carotid and comparatively pun- blood. id and arms the impure blood is le superior vena cavi auricle; , propelled, 1 .cut r-e-i,- : and 1 ventricle, into the pulmonary arti I he lungs, ami be oxygenized; but in the foetu passes through the into the oi t he descending aoi ta, where it mixes with that portion 1 re blood which ubclavian arteries, Some of this mixed Id 1 ia distributed he externa I iliai to I he lower ex tremities. while the remainder 1 1 portion) is conveyed by the hypogastric or umbilical Almost im- iately after birth, the foranu n imes I by ii im ■ layer, and the dui and duCI 1 nd aerate into impen inns fibrous cords. reviously to the acl of inspiration, are and solid in structure, and of a deep red color, and lie far back in 1 he che I 1 hi ii spe cific gravity is greater than water, in which they (or portions of them) consequently -ink. whereas lungs, or portions of lungs, thai have respired, flout in that Quid. children born in the seventh t ih of gesta tion a ile of living, although they usuall] ire much care ; and chil I be horn nine at a in period after the beginning of the sixth month, or even in some instances earlier than the sixth, but this is rare, and if born liv- ing, they commonly die soon after birth. Gestation; Embryo; Embryology, Human. FOG, or MIST (Dan. fog, led. fok, spray, from fjuka, t" he driven by the wind). Any- thing that obscures the clearness of the atn but specifically in meteorology the obscura- tion caused by the very minute globules of water floating in 1 he air. Because of their minuteness, the fog particles have not sufficient weight to descend rapidly in the presence of thi phere; thej are not hollow vesicles or spheres having an inherent buoyant gas, as was once supposed, hut are upheld by the slightest cur of air: the slowness of their progress down- ward is due mostly to the viscous resistance or internal friction of the air. These particles of water represent the condensation of vi '.as invisible a short time before. This densed into fog particles not by coming in con tact with or by passing ler bodies, but either by radiation of heat or by expansion and dynamic cooling. The latter is more frequent in the case of high fogs, but the former in I ea-e of the ordinary fogs at sea level. In general, when a mass of warm moist air rolls over cold land or cold water it radiates its warmth down-
