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turn, or when the cavity in one bone moves upon an emi- nence in another, as the bafes of the firft phalanges on the heads of the metacarpal bones. The planiform Diartbrofn is when the articulated bones flip upon one another, much in the fame manner as when we rub the palm of one hand upon the palm of the other. This articulation is found in the bones of the carpus and tarfus, and in the oblique procelTes of the ver- tebra:. The antients called the firft of thefe two kinds of ar- ticulation enarthrofis, the other arthrodia ; and fome modern French writers feem to comprehend both under the word ge- nou, a term borrowed from workmen, who probably firft ig- norantly took it from the human body to apply it to their in- ftruments. This term indeed, as they ufe and explain it, a- grees well enough with all the degrees of the orbicular Diar- tbrojis ; but there are undoubtedly many articulations of the other kind fo very flat, that a skilful workman would not ap- ply the term genou to fuel).
The alternative or reciprocal Diarthrofis bears fome refemblance to a hinge, and for that reafon the antient Greeks called it ginglymus. This has been divided into feveral kinds, but properly fpeaking there are only two. The firft is that which is confined to flexion and extension, and as in one of thefe mo- tions, the two bones always make an angle, it may be not im- properly termed the angular ginglymus : this is exactly the fame with the motion of a hinge. The fecond kind is adapted only to fmall turns toward each fide, or to final! lateral rota- tions, in the language of anatomifts : this therefore may be termed a lateral ginglymus. In each kind feveral differences are to be taken notice of. In the angular ginglymus, either each bone partly receives, and partly is received by the other, there being reciprocal eminences and cavities in each, as in the articulation of the os humeri with the ulna. Or there arc only a number of eminences in one bone received into a like number of cavities in the other, as in the articulation of the os femoris with the tibia. The lateral ginglymus is either fingle, as in the articulation of the firft vertebra of the neck with the apopbyfis dentiformis of the fecond ; or double, that is, in two different parts of the bone, as in the articulation of the ulna with the radius. It muft in general be obferved con- cerning thefe kinds of articulations, that fome of them are more perfect and clofe than others, and that they are not all confined to flexion and extenfion, or to the reciprocal turns already explained.
The obfeure Diarthrofis, or that which admits only of fmall motions, is alfo of different kinds, examples of which are found in the articulations of the bones of the carpus and meta- carpus, and of the fibula with the tibia. This articulation has by fome been called doubtful or neutral, and by others amphi- arthrofis,while others have reduced it to a fynarthrofis. The firft of thefe names might pafs, but the reft are improper. Win- few's Anatomy, p. 152.
D1ASCHISM, Diafcbifma, in mufic, the difference between the comma and enharmonic diefis. This is exprefled by the ratio i=H> for it} -|4=£ttt- Mr. Euler a bas taken this term in the fenfe here mentioned. Others call it the leffer comma b . The octave contains 61 Diafcbifms nearly, [a Tentam. Nov. Theor. Mufic. p. 107 and 112. b Ibid. p. 107.] See Comma, Cyd.
DIASIA, A/cewa, in antiquity, a feftival at Athens in honour of Jupiter, furnamed Metbiyj®-, i. e. the Propitious. Pott. Arcbreol. 1. 2. c. 20.
DIASTASIS, a word ufed by the writers in medicine in many different fenfes. It is frequently ufed as a name for that fepa- ration of the bones, when they naturally recede from one another. Sometimes it is ufed as the term for an interftice, as that between the ulna and radius, or between the tibia and fi- bula. Sometimes it is put for a diftenfion of the mufcles in con- vulsions ; and, when applied to the ftomach, it is made to fig- nify an inclination to vomit.
DIASTEM, cOrtr-ti««, in the antient mufic, was the difference of tenfions \ It anfwers to what we call an interval. Ariftoxenus b enumerates many differences of intervals : fuch as greater, or lefs ; confonant, ordiffonant; compounded, or uncompounded ; related to one genus, or to another ; laftly, rational or irrational. \* Arijlox. ap. Wallis. Append, ad Ptolem. Harmon, p. 154. D Id. ibid. p. 16. Edit. Mcibom. JVallh^ ibid.]
DIASTOLE (CycL)— Authors, who have treated of the heart, have all contented themfelves with accounting for its fyftole on rational principles, and have been fatisfied with referring its Diajiole to no other principle than a motion of reftitution. But Dr. Drake has thought more deeply on this fubject, and has found that refpiration has an effect upon the heart in this fenfe which had not been underftood before. The fyftole being the proper motion of the heart, a ftate of contraction by means of that fyftole muft needs be its natural ftate, and confequently no motion of reftitution natural, and without external violence it could have no Diajiole at all.
This will appear more plain, if we confider the circum- ftances of the heart, and its motion as a mufcle, with refpedt to other mufcles, That contraction is the proper action and natural ftate of all mufcles, is evident from experience as well as from reafon: for, if any mufcle be freed from the power of
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its antagonift, it is immediately contracted ; and is not bv'any action of the will or fpirits, to be reduced to a ftate of dilata- tion: thus, if the mufculi flexores of any joint be divided the extenfores of that joint, the oppofite muicles to the flexores, being freed from the contrary action of their antagonifts will exert themfelves; the joint will be extended without the con- fent of the will, and it will continue in that pofture: and on the other hand, if the extenfors are divided, the contractors will of themfelves exert their force, and the limb will be con- tracted without the confent of the will, and it will continue in this pofture. It is very evident from all this, that the mufcles of the human body have no reftitutionary power of their own but that all power of this kind in them is derived from the action of their antagonifts by which they are balanced : thus likewife the fphincters of the gula, anus, and vefica, havino- no proper antagonifts, are always in a ftate of contraction and fuffer nothing to pafs through them, but what is forced on by the action of other ftronger mufcles; which, though not properly antagonifts to thefe, yet, on all neceflary occa- sions, perform the office of fuch.
That the heart is a mufcle furnifhed and provided for motion like other mufcles, has been demonftrated, beyond all contra- diction, long ago, by Lower, and many others, who have written of it, and it is a folitary mufcle: it has no antagonift, and is not under the direction of the will : as it performs alfo no voluntary motion, it in all things approaches more to the na- ture of the fphincter mufcles, than to any kind bciide ; but, in its conftant actions of contraction and dilatation, it differs ex- ceedingly from all the mufcles of the body. This reciprocal mo- tion of the heart has given the learned abundance of trouble, who finding nothing peculiar in the ftructure which fhould occafion it, nor any antagonift whofe relation fhould produce it, have been extremely perplexed to find out the caufe of it. Lower having proved the heart a mufcle, and eftablifhed the manner of its mufcular motion, takes notice of no farther af- fiftance that it receives for the performing this, than what it has from the brain, by means of the eighth pair of nerves. Borelli, in his Animal Oeconomyi computes the motive power of the heart, to be equal to, at leaff, that of a weight of three thoufand pounds. The obftacles to the motion of the blood, through the arteries, he efteems equivalent to one hundred and eighty thoufand pounds, which is fix times as much as he rates the force of the heart at ; then deducting forty five thou- fand pounds, for the adventitious help of the mufcular elaftic coat of the arteries, he leaves the heart, with the full force of three thoufand pounds to overcome the refiftance of a hundred and thirty- five thoufand pounds; that is, with one, to remove forty-five. This ftupendous effect, he fatisfies himfelf with afcribing to the energy of percuffion ; but, had he proceeded, in his calculation, to the veins ; which, he allows to contain conftantiy a quantity of blood, quadruple to the contents of the arteries, and to which this energy of per- cuffion does not either reach at all, or but very languidly; he might probably have feen a neceffity of fome other expe- dient, to remove fo infuperable a difficulty. But not to infift regularly on the exactnefs of this computation, we may allow a much greater deduction than would be jufti- fiable, without leflening the difficulty. Dr. Lower, not- ftanding the care and iagacity with which he examined this fubject, feems to have overlooked fomething of very great moment, in the explication of the action of the heart ; for, though it fhould be granted, that the mufcular fibres of the heart, acted by the nerves, are the immediate inftruments of its fyftole or contraction, yet it muft not be denied that the intercoftal mufcles and diaphragm are of great fervice, to aid and facilitate this contraction, by opening a pafla<*e for the blood through the lungs, which denied, would be an invin- cible obftacle : neither do the lungs promote the motions of the heart in that way only, but the manner in which they af- fift the heart in its contraction, will appear manifeftly, if we confider the different pofture, fituation, and capacity of the blood veffelsof the lungs, in the feveral times of elevation and depreffion of the coftse.
The pulmonary artery arifes from the right ventricle of the heart, and runs in one trunk, till it comes to the afpera arte- ria, where it is divided, and fends a branch along with each divifion of the afpera arteria ; according to all the minuteft fubdivifions of which, it is alio fubdivided, accompanying all the bronchi in their paffage through the lungs. The pulmonary vein, which empties itfelf into the left ven- tricle of the heart, fpreads itfelf on the afpera arteria and bronchi, and continues its progrefs in the fame manner, in which the artery does. The neceflary confequence of this difpolition is, that this artery 'and vein, being co-extended with, and faftened to the bronchi, muft needs fuffer fuch al- teration of fuperficial dimenfions, as the bronchi do in the elevation and depreffion of the coftas ; while the ribs are in a ftate of depreffion, whether before commerce with the exter- nal air, or after the annular cartilages of the bronchi, fink into one another, and by that means their dimenfions are exceed- ingly contracted ; in conformity to this condition of the bronchi, the pulmonary artery and vein muft likewife either
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