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other furface which is toward the fcetus. Hence it is to be obferved, that the uterus appears not to be covered by any membrane within, and that the placenta alfo has none on that fide where it adheres to the womb, Co that there is nothing to prevent the blood from readily flowing from the uterus into the placenta, and thence to the child. What we ufually un- derftand by the word membrane is a covering of fo clofe a texture, as to keep out air, or fluids ; and it is very evident to reafon, that the placenta ought to have none fuch on that part where it is to communicate with the uterus; neither can the eye difcover any fuch there, though on the other fide it is eafy to diftinguifh one.
The navel-firing, befidc its vein, and its two arteries, is com- pofed of a fpongy fubftance, in which thefe blood veflels are lodged j this fpongy fubftance is compofed of a number of cellules which communicate with one another, and contain a glutinous liquor, which is ufually found in them in great abundance : if we confider the navel-jiring as a cylinder, the blood veflels run in a fpiral round its axis, and are laid one over another, but that differently in different fubje&s ; hence it is that the navel-jiring is tortuous.
When the veflels have run through the whole length of the navel-firing in the fame diameter, they break into a number of branches, and enter the placenta, where they afterwards di- vide themfelves into capillary ramifications. The diameter of the vein is all the way double to that of both the arteries ; fo that it contains twice as much blood as the two arteries toge- gether. If the navel-firing were only compofed of thefe three veflels, the feetus, in moving, might eafily comprefs them in fuch manner, as to ftop the current of the blood, and its in- evitable death muft be the confequence of this; but nature having provided this fpongy, or cellular fubftance, for their lodgement, this is able to give way to them, when preffed in any direction, and enables them, therefore, to bear a confider- able force, without the current of the blood being flopped by them in it; nor is the glutinous liquor, of which thofe cells are full, of little ufe in lubricating thefe veflels. Mem. Acad. Par. 17 14.
Navel-m-wi, cotyledon, in botany, the name of a genus of plants, the characters of which are thefe : the flower is com- pofed of one leaf, fafhioned into a bell-fhape, with a long tube, and divided into feveral fegments at the end: from the center of the cup there arifes a pillil, which perforates the bottom of the flower, and ripens afterwards into a fruit compofed of feveral fmall {heaths of capfules, collected into a fort of head, and all opening at their tops, and ufually containing an extremely mi- nute feed.
The fpecies of cotyledon enumerated by Mr. Tournefort are thefe: 1. The common kind called wall navel-wort, and um- Ulicus veneris. 2. The great Portugal kind. 3. The yellow- flowered kind, with a long, tuberofe, and creeping root. 4. The middle-fized cotyledon, with a yellow flower. 5. The fhrub, African cotyledon, with round leaves, bordered with ai edge of purple, ufually called the round-leaved, hoary, Afrr can jedurn, or houfeleek, 6. The thick, or cylindric-Ieaved, African cotyledon, with beautiful purple flowers, known alfo many under the name of a fedum, or houfeleek. 7. The lon^ leaved, African cotyledm, with fibrofe roots, and umbellated flowers, 8. The fibrofe- rooted, fea cotyledon, with houfeleek leaves, and flefh-coloured flowers. Thefe are the genuine fpecies of cotyledon, but authors who have lefs accurately exa- mined plants, have ranked under this name feveral which are fpecies of houfeleek, of geum, and of faxifrage. Town, inft, p. 90.
The cotyhdom, when not in flower, may ufually be known by having thick, tuberofe roots, and thick or fatty roundifh leaves. Thefe, however, are found to be no certain charac- ters, as there are plants truly of this genus, which have long leaves, and fibrofe roots.
NAVETTE, the name by which moft of the European nations call the napus, or bunias fylvejlrh of Lobe], the plant from the feeds of which the oil which we call rape oil, and the French and others navette oil, is exprefled. The plant is cultivated in many parts of England, and in other countries, for the fake of the Cccd ; the oil drawn from this is ufed by many artificers and is of a fine yellow colour, and the fmcll is not offenfive. Lemery DicT. of Drugs.
NAVICULA, in natural hiftory, a name given by a late French author to the clafs of fliells called by us the nautili. See Nautilus.
NAVIGATION {Cycl.) — Sub marine Navtcation, or the art of failing under water, is mentioned by Mr. Boyle as the deflderatum of the art of navigation. This was attempted, and fuccefsfully, according to him, by Cornelius Drebbel • feveral perfons who were in the boat breathing freely all the time. B-ylc's Works, abr. Vol. 1. p. 130.
NAVIGATOR, in the fea language, ufually denotes a fierfon capable of carrying, or guiding a fhip to any place defired.
NAULIUM, in antiquity, a muflcal inftrument otherwife called nahlum. See Naelum.
NAULUM, among the Romans, properly fignified freight; whence it is ufed to denote a piece of money put into the mouth of a perfon deceafed, to pay Charon the ferry- man tor his pafiage : this piece was to be of the current coin of
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the emperor then reigning ; fo that from it the time of the perfon's death may be known. Danet. in voc. NAUPLIUS, in natural hiftory, a name by which fome authors have called the nautilus, as well the papyraceous, as the cham- bered kind. See Nautilus. NAUTICUS {Cyd.) — Nauticus, in natural hiftory, a name
given by fome to the nautilus. See Nautilus. NAUTILUS, {Cycl.) in natural hiftory, the name of a genus of fhell-fifh, the characters of which are thefe : it expreffes, in general, in every fpecies, the figure of a boat, or veffel, made for fwimming on the water ; but in the different fpecies, it is of very different figures, roundifh, or oblong, thin, or thick, fur- rowed, or fmooth, and fometimes is auriculated, fometimes not. Bonani obferves that this genus of fhell-fifh is very well named from the Greek morlA©., which fignifies both a fifh, and a failor, for that the fhells of all the nautili carry the appear- ance of a boat, or fhip, with a very high poop. Different authors, among the antients and moderns, have called the nautilus by the names of pompilus, nauplius, nauticus, ovum polypi, polypus UJlaceus, and the French call it k voilier. It is fuppofed that men firft learned the method of failing in veflels, from what they faw pradf ifed by this creature. We at prefent know two diftincf genera of nautilus, the thin and the thick-fhelled nautilus. The firft is called nautilus pa- pyraceus, and its fhell fcems indeed no thicker, or ftronger, than a piece of paper, when out of the water. This fpecies is not at all faftened to its fhell, but there is an opinion as old as the days of Pliny, that this creature creeps out of its fhel], and goes on fhore to feed. When this fpecies is to fail, it ex- tends two of its arms on high, and between thefe, fupports a membrane, which it throws out on this occafion ; this ferves for its fail, and the two other arms it hangs out of the fhell, to ferve occafionally either as oars, or as a fteerage ;.but this laft office is generally ferved by the tail. When the fea is calm, it is frequent to fee numbers of thefe creatures diverting themfelves with failing about in this manner ; but as foon as. a ftorm rifes, or any thing gives them difturbance, they draw in their legs, and take in as much water as makes them fomc- what heavier than the fea water in which they fwim, and they then fink to the bottom. The manner of their voiding this abundant water when they would rife again, is by a number of holes, of which their legs are full. Hiftoir. Natur. Eclair. Part. 2. p. 218.
The other nautilus, whofe fhell is thick, never quits that ha- bitation. This fhell is divided into forty, or more partitions, which grow fmaller and fmaller as they approach the extremi- ty, or center of the fhell : between every one of thefe cells, and the adjoining ones, there is a communication, by means of a hole in the center of every one of the partitions. Through this hole there runs a pipe, which is of the whole length of the fhell. It is fuppofed by many that the fifh occafionally changes the place of its residence, by removing from one to another of thefe cells, through this pipe in the little hole ; but this is by no means probable, as the body of the fifh muft ne- ceffarily be crufhed to death, if it were poflibleto make it pafs through fuch a hole. It feems, therefore, much more proba- ble, that the fifh occupies always the largeft chamber in its fhell, that is, it lives in the cavity between the mouth and the firft partition ; and that it never removes out of this, but that all the apparatus of cells, and a pipe of communication which we fo much admire, ferves only to admit occafionally air, or water, into the fhell, in fuch proportion as may ferve the creature in its intentions of finking or fwimmin». Rum- pbius, p. 17. fig.b.
Ariftotle has evidently defcribed two fpecies of nautili, and fome authors have thought he defcribed three; of this number is Bellonius, whom Rondeletius treats very feverely for the er- ror of this opinion; but it is cuftomary with Rondeletius to be fevere upon Bellonius. Bonani, p. 89.
Some authors call this fhell csneba margarltifera ; hut this can be only on occafion of the fine colour of its infide, which is more beautiful than any other mother of pearl ; for it has not been obferved that this fpecies of fifh ever produced pearls. It muft be obferved that the polypus is by no means to be confounded with the paper-lhelled nautilus, notwithftanding the great refemblance in the arms and body of the enclofed fifh ; nor is the cornu ammonis, fo frequently found foflile, to be confounded with the thick-fhelled nautilus, though the conca- merations, and general ftruflure of the fhell is alike in both, for there are great and effential differences between all thefe genera. See the article Cornu Ammonis, &c. The three principal differences of the nautilus clafs are, that fome are papyraceous, fome are auriculated, and fome are umbilicated. Ronddct. p. 98.
The feveral known fpecies of nautilus are thefe : 1. Tire great, fmooth, and thick nautilus. 2. The little nautilus, with a fmooth and thick fhell. 3. The fmooth, and thick, arobilk. cated nauti.'us. 4. The common, concameratcd nautilus. 5. The furrowed, and empty -nautilus, with no diaphragms, or reparation within. 6. The thin, and flatted, papyraceous nautilus. 7. The auriculated nautilus, with a wider fhell. 8. The undulated, and furrowed nautilus, with dents on each fide the edge. 9. The nautilus with a ridged and ferrated backj This laft fpecies is often found in the cabinets with-
