Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/743

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aperture ot its' (hell, when it pleafes, certain bodies which re- femble not a little the legs of ftar-fifh ; but thefe ferve not at all to its motion ; but, on the contrary, their real ufe is to keep the creature ftill, and fixed in the fame pofition ; and, to defcribe them more exactly, they very aptly refemble the horns of fnails ; whence Mr. Reaumur has chofen rather to call them horns than legs. The ufe the Urchin makes of thefe horns, while it is in motion, is to feel about, and try the ground on which it marches ; and they ferve the creature as a ftaff does a blind man in his walking, to touch and try every thing that lies in the way ; and to make them ferve to this purpofe, it is continually extending or retracting them during the time it is moving. Thefe horns are not only placed about the orifice of the fhell, but they are every where difperfed among the fpines, all over the furface of the fhell. To underftarid properly the pofition of thefe hums, it will be neceflkry to confider the figure of the fes-Uribm fhell, as we ufually fee it, that is, ftripped both of its (pines and its horns. In this ftate it appears a very beautiful piece of workmanfhip. It is a hard body, the figure of which approaches to that of a iegment of a fphere, or a mould of a button hollowed within ; it has an aperture at the very fumm.it of the fhell, and another at the bafe, juft oppofite to it ; this is the cafe in the common kind ; for there arc great varieties in the place of thefe holes, in the different fpecies : at this lower aperture is placed the mouth of the animal ; the upper is fuppofed to ferve it to dif- charge the excrements by. The whole external furface is co- vered with protuberances of different fizes, but all difpofed in a beautiful and regular order ; they divide the whole furface, as it were, into ten fphcric ifofceles triangles, all which have their fummit at the upper aperture, and their bafe at the lower. Of thefe there are five large and five fmall ; but all the large ones, and all the fmall ones, are regularly of the famefize one with another. There is a fmall band, which is lefs rough than the other parts, which feparates every larger from every (matter triangle ; thefe bands are alfo in themfelves fo many triangles ; but the calling them by this name, wilJ better ferve to diftinguifh them from the others. As the tri- angles, both large and fmall, are all covered with eminences cr protuberances, tfiefe bands are, on the contrary, all pierced with fmall holes; thefe run quite through the fhell, and their apertures are more fenfibie on the infide than on the out. Thefe holes are beautifully diftinguifhed, on holding the fhell againft the light, and are arranged in a very beauti- ful order, ?nd in regular feries on every band. There are two forts of ranges of thefe upon every band, one fort con- taining two holes all the way, and the .other four; thefe are placed in a regular alternate order; firft, a range of two, then a range of four, then another of two again, and fo on to the extreme edge of the band.

The fpace contained in every tiiangle is alfo in the fame man- ner divided into feveral parts by a, number of lines, which be- gin at the upper aperture of the fhell, and terminate at the lower ; but as thefe lines in the bands are marked by little perforations, they are, in the triangles, made of thofe emi- nences which render the furface of the fhell rough and un- even ; thofe eminences which are in the middle of every line, are larger thin thofe which come towards one or the other end ; and the feveral different lines are made up of thefe emi- nences of different fizes one from another, fo that, on the whole, there is every where a great variety of them. Every one of thefe eminences refembles a fort of teat or nipple, with a portion of the breaft ; or, to give a more determinate idea of them, each is a portion of a fphere, the fummit of the convexity of which is crowned with another portion of a much fmaller fphere. Thefe are the parts on which the bafe of every (pine of. the Urchin is fixed; and as this bafe is hol- low, it naturally envelopes the fmall portion of a fphere at the fummit of each eminence, and is able to move and turn ahy way upon it, in the manner of the ball and focket articula- tions. The fmaller eminences fuftain the fmaller fpines, and the larger the great ones ; the number of thefe eminences, or, in other words, the number of the fpines on one fifh, is fur- prizing ; many of them are fo fmall, that it is not eafy tc. count them regularly; but Mr. Reaumur found, in general, that there were confiderably more than two thoufand on every fifh ; nor is the number of the perforations before-mentioned lefs to be wondered at ; they take up but a very fmall fpace on the fhell, in proportion to the fpines, yet Mr. Reaumur ob- ferves, that their general number in every fhell cannot be lefs than thirteen hundred. From knowing the number of thefe perforations, we are able to determine that of the horns ; for every horn has its origin from one of thefe. The horns are never feen all together," they are only vifible when the creature is in the water, and even then it only exerts fome of them at a time, fo that it is not to be expected that they fhould all be feen together. When it is in motion, it only fliews thofe which are placed on that fide of the fhell which is to move forwards in the journey ; and when it is at reft, we only can fee thofe which it thrufls out, and fattens to ftones or other bodies, to fix itfelf by. Thefe ferve by way of anchors to the creature, it glews them faff down to the ftones, C3V. and if the creature be forcibly removed, they are generally, in part, if not entirely, broken and deftroyed by it. When the

creature is taken out of the water, they are no more to be difcerned, they are bent and folded together, and are in a flac- cid ftate, fo that nothing but their ends can be at all per- ceived, and thefe no body can tell what to make of, who has not before feen them in their exerted ftate in the water. The fpines are ail capable of affifthig the creature in its mo- tions, but thofe it principally employs are fuch as are placed near its mouth, as thefe can turn upon their balls eyery way with equal facility, the creature finds it equally eafy to move on any fide ; and when it has determined which way it will move, thofe fpines which ftand directly toward that point, and thofe which are directly oppofite, are of equal fervice to it ; it draws itfelf forward by means of the firft, and pufhes itfelf on with the others ; to do this, it firft thrufts out the foremoft ones as far as poffible, and preffing them againft the bottom, it draws on its body by them ; and this is fucceeded by its drawing up the hinder ones clofe to its fhell, and then fixing them againft the bottom, it pufhes itfelf forward by them. This is the manner of this little creature's marching in the common way with its mouth downward ; but it has this ftrange Angularity, that it is not confined to this pofture alone in marching, but can, with equal eafe, walk with its mouth upwards, or run along fideways in the manner of a wheel ; or in any direction between thefe. The legs and the horns cover all parts of it, and are in every part of it equally able to move it every way. What a prodigious number of mufclcs nluft this little creature have to be able to move feparately thirteen hundred horns, and more than two thoufand fpines, which ferve for legs. Mem. Acad. Par. 1 7 1 2.

UREDO, a word ufed by fome of the chemical writers to ex- prefs the virtues of metals communicated to them from the fun. Pliny ufes the fame word to exprefs the fmut affecting fruits ; and fume medical writers have exprefled by it a very violent and excruciating pain in the head. See the articles Smut and Blasts.

URENA, in botany, a name given by Dillenius to a genus of plants, the characters of which, according to Linna;us, are thefe: The cup is a double perianthium ; the external one is compofed only of one leaf, divided fiightly into five broad fegments ; the inner one is compofed of five narrow and an- gular leaves. The flower is compofed of five oblong petals, which are broadeft at the apex, and end in an obtufe point, and at their bafes are narrow and grow together. The ita- mina are numerous filaments, which at their bafes grow toge- ther into a cylinder, but ftand free at the tops. The apices are roundifh. The germen of the piftil is roundifh ; but it is formed into five angular prominences. The ftyle is fimple, and of the length of the ftamina, and is crowned with ten ftigmara, which are headed, reflex, and hairy. The fruit is a roundifh pentangular echinated capfulc, being compofed of five valves, and containing five cells. The feeds are fingle and roundifh, and at their ends fomewhat compreffed. Lin- nai Gen. Plant, p. 329. Dillen. Hort. Eltham. p. 319.

URETHRA (Cycl.) — The hard knots which are fometimes formed in the corpus cavernofum UretbrS, after venereal vi- rulency, are very difficult to cure. Till the pocky matter is all extirpated, it is 111 vain to attempt the cure of thefe knots, and even then they do not yield to mercury in any fhape, but are refolved by embrocations of the waters of Bareges. See Mem. de l'Acad. de Chirurg. Tom. x. Mr. leCat has given us the figure of the canal of the Urethra* determined by folid injections. See Phil. Tranf. N°. 460. Sect. 5. p. 684.

Urethra Depreffor, in anatomy, a name given by Santorini to a mufcle of the pudenda in women, the fame with that which he calls in men the elevator Urethra and ejaculator. Winflow, and the generality of the French anatomifts, call it the profiatique inferieur. And Albinus, who has given to the tranfverfaiis penis the name of the tranfverfus perinai, calls this alfo the tranfverfus alter per inai.

U re thrje Elevator, in anatomy, a name given by Santorini to a mufcle of the penis, which he calls alfo the ejaculator ; it is called by Albinus the tranfverfaiis alter perinai, to diftin- guifh it from the mufcle commonly called the tranfverfus pe- nis ; but which he calls the tranfverfus perinai. The fame mufcle in women is called the depreffor Urethra by the fame Santorini.

UretHRAM Dilaiam, in anatomy, a name given by DeGraef to a mufcle of the penis called the Vrethram trahens by Spi- gelius, and by the later writers, the accelerator penis. See the article Accelerator.

Urethram Trahens, in anatomy, a name given by Spigelius and others to a mufcle now generally called the accelerator. See the article Accelerator.

Urethrje Triangularis, in anatomy, the name of a fuppofed mufcle of the penis, called alfo dilatator penis, and dilatator^ Urethra-, it is properly only a procefs of the fphincter ani continued into the perinseum.

URIBACO, in zoology, the name of a BrafUian fea-fifh, efteemed a very well tafted and wholfome one. It is fome- what of the figure of the pearch ; its back is ridged, and its belly fomewhat protuberant. It grows to ten or twelve inches long. Its teetlvare fmall and fliarp, and the ends of its gills terminate in a triangular point. Its- gill-fins end in a

triangular