FOE
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FOE
Then, making C R to C O, as s to r for the Immerfion of a Ray; or as r to s for the Emerfion (J. e. as the Sines of the Angles in the Medium which'the Ray enters, to their correfpondins Sines in the Medium out of which it comes) and laying C R, from C towards O, the Point R will be the fame tor all the Rays of the Point O. Laftly, drawing the Radius P C, if need be, continued ; with the Centre R, and Diftance O P ftrike apiece of an Arch, interfering V C in Q. The Line Q^R being drawn, mall be parallel to the refracted Ray; and PF being made parallel there- to, ftiall interfect the Axis in the Point F, the Focus fought.
Or, make it, as C CL: C P : : C R : C F. Then will C F be the Diftance of the Focus from the Centre of the Sphere.
This Author gives a Demonstration of the Method ; and adds various Figures, exhibiting the various Cafes of R'iys either diverging, or converging, as they enter, or emerge out of the Surface either of a convex or concave Lens.
From this Principle all the Rules for the Foci of Rays parallel to the Axis, as likewife for the principal Focus, where the Rays neareft the Axis do unite, are dedue'd. As,
Hence i° If O P be equal to C R ; the Points Qjmd C are coincident, and the Rays O P, after Refraction, run on parallel to the Axis. 2. If the Point Q^fall on the fame Side of the Axis, as is the Point P 5 then the Beams after Refraction do tend on, either Diverging, or Converging, as before : But if Q^ fall on the other Side the Axis,- the Diverging Rays are made to converge by a Convex, or the Converging to diverge by a Concave Glafs. 3 If O P do exceed C R, the Focus is in all Cafes on the fame Side of the Glafs, as is the Centre of the Sphere C But contrary- wife, if O P be lefs than C R, the Focus falls on the other Side of the Glafs beyond the Vertex V. 4. An Object may be fo placed, that the Rays next the Axis of a Con- vex Glafs iliail have an imaginary Focus tranfmitting di- verging Rays, when the more remote Parts thereof mall make them converge to a real Focus. 5 If O V, the Di- ftance of the Object from the Pole or Vertex of the Glafs, be taken inftead of O P, then will C Q_bethe Difference of O V and C R ; and as that Difference is to C R, fo is the Radius C V, to C F, the Diftance of the principal Fo- cus from the Centre of the Sphere, whereof the Glafs is a Segment. Or elfe, as C Q_ : to O P or R Q_ : : So P C : to V F, the focal Diftance from the Pole of the Glafs. Whence follows a General Rule for the Foci of all Glaffes ; only according to Cor. 3. if O V do exced C R, the Focus is on the fame Side of the Glafs, as the Center of the Sphere : But if C R be greater, then -the Focus is on the oppofite Side of the Glafs: Whence it will be determined, whether the Focus be real, or imaginary.
What has been faid of one Surface of the Lens, is eafily applicable to the other, taking F the Focus, for an Object.—
FODDER, any kind of Meat for Horfes, or other Cat- tle : In feme Places Hay and Straw mingled together, is ac- counted Fodder :
In the Civil Law it is us'd for a Prerogative that the Prince has, to be provided of Corn, and other Meat, for his Horfes, by the Subjects, in his warlike Expeditions.
FOD1NA, a Name fome Authors give to the Labyrinth in the Bone of the Ear. See Labyrinth.
FOECES, -) r F;eces.
FOECAL, KSec) Fjeces.
FOECUL^Ej 1 F*culj£.
FOETOR, in Medicine, ftinking, or fectid Effluvia, pro- ceeding from the Body, or the Parts thereof. See Efflu- via.
Fosters arife from ftagnant, extravafated, corrupted, or poifon'd Humors^ as alfo from any thing capable of atte- nuating and volatilizing the Oil and Salts : As, Abftinence, Heat, too much Motion, Acrimony of Food, &C.
Foetor Narium, Stench of the Noftrils, a fort of Dif- eafe, arifing from a deep Ulcer, within fide theNofe, yield- ing a fcetid Smell. Its Caufe, according to Galen, is a Jharp Humor, falling from the Brain upon the Mamillary ProcefTes. This is one of the Caufes that annuls Marriage. See Di- vor c e .
FOETUS, in Medicine, denotes the Child, while yet contain'd in the Mother's Womb: but particularly, after it is perfectly form'd : Till which Time it is properly call'd £mbryo. See Embryo.
The manner of the Conception, or Generation of the Fcetus, is Matter of great Conrroverfy.
That all the Parts of the Animal did exift, and that its Fluids were in Motion, before Generation, is generally al- low'd ; but whether the Animalcule was lodg'd in the Male, or Female, is not agreed of.
Many of the Moderns will have the Ova, or Eggs, con- tain'd in the Ovary of the Female, to be the firft Matter, or Stamen of the Fcetus : Thefe Eggs, they fuppofe to contain all the Parts of the Fcetus in little ; and that being impreg- nated with the Male Seed, the Parts thereof become en-
larg'd, and difplay'd : From the Ovary thty ire convey'd by the Fallopian Tubes into the Uterus, where they receive their Impregnation, Accretion, ££Tc. See Egg, and Ovary.
Others will only have the female Ovum to be a proper Nidus for the Animalcule, which, they contend, is in the Male Seed. The Animalcule getting into an Ovum fit to receive it; and this falling thro' one of the Tubas Fallopiana: into the Womb 5 the Humors which diitil thro' the Vefl'els of the Womb penetrating the Coats of the Egg, fwell and dilate it, as the Juice of the Earth does Seed thrown into the Ground. Or elfe, the Branches of the Veins and Ar- teries, whereby the Egg was tied in the Ovary, being broken, knit with the VefTels of the Womb. See Gene- ration, Conception, Seed, £fc.
The firft thing that appears of a Fcetus t \s the Placenta, like a little Cloud, on one Side of the external Coat of the Egg -. About the fame time the Spina is grown big enough to be vi- fible ; and a little after, the Cerebrum and Cerebellum appear like two fmali Bladders: Next, the Eyes {land prominent in the Head ; Then the Punctum fallens, or Pulfat ion of the Heart is plainly feen. The Extremities difcover themielves laft of ail.
The Fcetus, when form'd, is almoft of an Oval Figure, while it lies in the Womb : For its Head hangs down, with its Chin upon the Breaft : Its Back is round 5 with its Arms it embraces its Knees, which are drawn up to its Belly ; and its Heels are clofe to its Buttocks : Its Head upwards, and its Face towards its Mother's Belly. About the ninth Month, its Head which was hitherto fpecifically lighter than any other Part, becomes fpecifically heavier ; its Bulk bearing a much fmaller Proportion to its Subftance, than it did.
The Confequence of this Change, is, that it tumbles in the Liquor which contains it : Its Head falls down ; its Feet get up ■ and its Face turns towards its Mother's Back. But being now in an irkfome Pofture, tho' at the fame time a favourable one for its Exit; the Motion it makes for its Relief, gives frequent Pains to the Mother ; which caufes a Contraction of the Womb, for the Expulfion of the Fcetus, See Delivery.
Indeed, what fome Anatomifts pretend to give of the Pofture of the Fcetus in the Womb, in the fcveral Stages of Geftation, is very precarious.
In the firft Month it is of no moment, how it lies in the Womb: In the latter Months, after the Fcetus is grown not only quick, but robuff, it frequently changes its Pofture of it felf ; as not only the Mothers themfelves feel, but any- other Perfon by laying the Hand on their Bellies, frequently may. However, its ordinary Poffure is fuppofed to be fit- ting : As the Time of Birth draws near, it turns it felf, and prefents the Head to the Os Uteri 3 tho' fometimes it of- fers the Feet firft, and fometimes lies a-crofs, and offers ei- ther a Hand, a Knee, or 'the like ; which are irregular Si- tuations, and without a deal of Adrefs in the Midwife in turning the Feetus, both the Mother and the Infant are in Danger. See Parturition, &c.
The Fcetus is enclofed in two Membranes, or Coats ; the inner, which immediately invefts the Fcetus, and the Liquor wherein it lies, is call'd the Amnios : The outward Mem- brane is called the Chorion. See Amnios, and Ciiop.ion.
In fome Animals there is a third Membrane, called the Alla?itois, whofe Place is between the other two ; and ferves for the Difchargc of the Urine of the Fcetus, brought hither by the Urachus.
Dr. Necdbam feems to have difcover'd fomething analo- gous hereto in the human Fcetus, and calls it the Membra- 11a Urinaria .- But others chufe only to make it a Duplica- turc of "the Chorion ; tho' the tNeceffity of fuch third Mem- brane be the fame in Men, as in Cows, Sheep, £5V. See Allantois.
There are fome Differences in the Structure, Mechanifm, and Proportion of the Parts of a Fcetus $ from thofe of an Adult: And even fome additional and extraordinary Parts; by which the Nutrition of this Zoophyte, or Plant Animal; and the Circulation of the Blood therein, are effected.
The principal Variations are about the Liver, Heart, and Lungs. Of thefe the moft considerable are the Umbilical VcJJels, which are two Arteries, a Vein, jand Urachus, arif- ing from the Placenta, and convey'd thro' the Navel to the Liver of the Fcetus, near the Navel ; which after the Birth drying up, become impervious and ufelefs. See Funicu- lus, and Umbilical VcJJels.
In the Liver it felf is an extraordinary Communication between the Porta and Cava, call'd Canalis Venofus, which after the Birth gradually dries up. In the Heart, at the Mouth of the Vena Cava, is the Foramen Ovale, whereby that Vein has a Communication with the pulmonary Vein: There is alfo a Communication between the Aorta, and Pul- monary Artery, by means of the Canalis Arteriosus, which paffes between the two, at about two Inches diftance from the Bafe of the Heart. 'Tis by means of thefe two Canals or Paffages, that the Blood circulates in the Fcetus while inclos'd in the Womb ; they fervingto convey and nafs the
Blood
