RHO 



It II O 



Window calls the inferior portion of the rhomboidalis, Al- 

 binos calls rhomboidauis major; and what he calls the upper 

 portion of that mufcle, Albinus calls rhomboidaeus minor. 

 Sec Rhomboideus. 



RHOMBOIUALIS. See Rhomboideus. 



RHOMBOIDES, in Geometry, a quadrilateral figure, 

 whofe oppofite fides and angles are equal, but which is 

 neither equilateral nor equiangular ; or, it is an oblique- 

 angled parallelogram. 



Such is the figure NOPQ, Plate XII. Geometry, jig. I. 

 For the method of finding the area of a rhomboides, fee 

 Rhombus. 



Rhomisoidl>, in Ichthyology. See CiIJETODON Argus, 

 Striatiis, isfc. 



RHOMBOIDEUS, in Anatomy, a mufcle of the 

 (boulder, fo called on account of its lhape ; dorfo-fcapulien 

 of Chan filer. 



It is a broad, flattened, quadrilateral mufcle, placed ob- 

 liquely at the lower part of the neck and upper part of the 

 back, and extending from the lower portion of the liga- 

 mentum nuchas, and the fuperior fpinous procefles of the 

 back, to the bafis of the fcapula. The whole of its fibres 

 go obliquely from the fpine downwards and outwards to 

 the fcapula ; thus as it is regularly quadrilateral, it has a 

 correct rhomboid figure. Its pofterior furface is chiefly co- 

 vered bv the trapezius ; the latiflimus dorii lies on a final] 

 portion of it below ; and between thefe two mufcles its 

 fibres are covered by the fkin only. The ferratus fuperior 

 polticus, fplenius, longiflimus dorfi, facrolumbalis, the ribs, 

 and the external intercoltal mufcles, are covered by its an- 

 terior furface. Its upper edge goes from the lower part 

 of the ligamentum nucha;, obliquely outwards and down- 

 wards, to the bafis of the fcapula oppofite to the commence- 

 ment of the fpine of the bone. In great part of its extent 

 this edge is in contact with the levator fcapula?. The lower 

 margin reaches from the fpinous procefs of the lourth or 

 fifth dorfal vertebra to the inferior angle of the fcapula. 

 The two edges arc nearly of equal length, and parallel to 

 each other. The internal edge is attached to the lower 

 portion of the ligamentum nucha;, to the lalt cervical 

 fpinous procefs, to the four or five fuperior dorfal fpines, 

 and to the interfpinal ligaments of thofe bones. The outer 

 edge is fixed to the bails of the fcapula, from its inferior 

 angle to above the origin of the fpine, between the fupra 

 and infra fpinatus behind, and the ferratus magnua in front. 



The rhomboideus is partly mufcular, partly aponeurotic. 

 Its inner edge is attached by aponeurotic fibres, having the 

 fame direction as the flefhy one, fliort above, and longer 

 below. It is fixed to the upper part of the fcapula by 

 fhort aponeurofes ; its infertian in the remainder of the 

 bafis is by an aponeurofis attached only at its upper and 

 lower extremitk ., ! pafiage between thefe points 



for branches of the tranfverfalis colli artery. The mufcular 

 are all directed obliquely from within outwards, and 

 from above downwan 



The mufcle is generally divided into two portion-, united 

 bv cellular tiffue to a fuperior fmaller, and an inferior larger 

 one: thefe are call d the rhomboideus minor and major. 



By drawing the bafia of the fcapula obliquely upwards 

 and back ■ the rhomboideus carries the inferior angle 



towards tlie fpine, and makes the anterior angle of the bone, 

 the Ihoulder, roll forwards and downwards. 

 Thus it i the fcapula to its fituation after the 



(houlder has been raifed. It concurs in this action with 

 the levator fcapula . 



RHOMBOID1A, in Natural Hi/lory, the name of a 

 genus of fpars, given them from their being of arhomboidal 



form. They owe this figure to an admixture of particles 

 of iron, and confill of fix planes. 



Of this genus there are only two known fpecies, -viz. a 

 white thin one with very thin cruris, and a whitifh-brown 

 thick one with thicker crufts. They are both found in the 

 foreft of Dean in Glouceflerlhire, and in other places, where 

 there are iron-ores. Hill. 



RHOMBUS, 'VojxSo:, is formed of fO/iSof, of pop&w, to 

 pafs or turn round, in Geometry, an equilateral rhom- 

 boid ; or a quadrilateral figure, whofe fides are equal and 

 parallel, but the angles unequal ; two of the oppofite ones 

 being obtufe, and the other two acute. Such is the figure 

 A BCD, Plate TL.Jig.ii. 



To find the Area of a Rhombus, or Rhom[/oiiies-—\Jpon 

 C D, which is here aflumed as a bafe, let fall a perpen- 

 dicular .V E, which will be the altitude of the parallelogram : 

 multiply the bafe by the altitude, the product is the area. 

 Thus, if C D be = 456, and A E = 234, the area will be 

 found 106704. 



For it is demonftrated, that an oblique-angular parallelo- 

 gram is equal to a rectangle upon the fame bafe C D, and 

 of the fame altitude AE (fee Parallelogram). But 

 the area of a rectangle is equal to the factum of the bafe 

 into the altitude ; therefore the area of an oblique-angular 

 parallelogram is equal to the fame. See Rectangle. 



The area of a parallelogram, rhombus or rhomboides, may 

 be found by means of the following proportion : As radius, 

 ;'. e. fine of 90 or tangent of 45 , is to the fine of any 

 angle of a parallelogram, fo is the product of the fides in- 

 cluding the angle to the area of a parallelogram : that is, 

 AD x D C x nat. fine of the angle D = the area. For 

 having drawn the perpendicular A E, the area, by the firft 

 rule, is A E x D C ; hut as rad. 1 (fin. < E) : fin. < D :: 

 A D : A E = fin. < D x AD ; therefore A E x DC = 

 DC x fin. < D x A D is the area ; or, 1 : fin. < D :: A D 

 x D C : fin. <DxADxDC = the area of the paral- 

 lelogram. 



N.B. As the angles of a fquare and rectangle are each 

 90 , whofe fine is 1, this rule is the fame as the former. 



E.G. 1. What is the area of a rhomboides, whofe length 

 is 36 feet, (lope-height 25.5 feet, and one of the lefs angles 

 58 ? Here rad. or 1 : .8480481 (nat. line of 58 ') :: 9 i8 

 {— 25.5 x 36) : 778.5081558, the area. Or, by tiling the 

 logarithms, rad. (lo.ocooboo) : fin. 58° (9.9284205) :: log. 

 of 918 (2.9628427) : 778.5081, the number correfponding 

 to the log. (2.S912632). 



2. What is the area of a parallelogram whofe angle is 90 , 

 and the including fides 20 and 12.25 chains? Anf. 245 



3. What is the area of a rhombus, each of whofe tides is 

 2 1 feet } inches, and each of the lefs angles t;$° 20' ? Anf. 

 362.208757 feet. 



4. How many acres are in a rhomboides whofe lefs angle 

 is 30°, and the including (idea 25.35 allt l 1C M '''■'■ 

 13 acres 29.12 perches. 



The area of either of the forementioned figures may alfo 



lie had by the following rule, which is ewmni.m to all qua- 

 drilaterals. As radius is to the line of the angle which the 



interfering diagonal of a parallelogram make with each 



other, lo is the product of the diagonals 1 d lublethe area: 



AC ■- B 1) ■ nat. fine ol the angle It 



that 



-- = tii 



N. B. Becaufe the dia fquare and rhombus in- 



terfedl at 1 righl angle, whofe line ia 1, halt the product of 

 their diagonals ia tb areaj thai 1 . j A C* in the fquare and 

 \ A C x B D in the rhombus is the area, 



E. G. 



