Page:Cyclopaedia, Chambers - Volume 1.djvu/971

 HEA

Sir Jfaac Nenvton carries the Thing fomewhat farther, and fuppofes Hearing, like Seeing, to be perform'd, not immediately by the Vibrations of the Air, but by thofe of fome other more fubtle Medium, excited in the auditory Nerves by the Tremors of the Air, and propagated thro' the folid Capillaments of the Nerve to the Place of Senfa- tion. See Medium, Vision, &c. '

The curious Structure of the Lab-rinth, and Cochlea, tend to make the weakeft Sounds audible; for the whole Organ of Hearing being included in a fmall Space, had the auditory Nerve run in a ftrait Line, the Impreffion would only have been made on a very fmall Part of it; and the Strength of the Impreffion being, Ceteris paribus, always as the Number of Parts upon which the Impreffion is made, Sounds which are now low, could not have been heard at all. — If the auditory Nerve had, like the Re- tina, been expanded into a large Web, which had covered or lined fome wide Cavity, the Impreffion of Sounds even in this Cafe had been much weaker, than they are now : For this large Cavity had given Room for the Sounds to dilate; and all Sounds grow weaker, as they dilate.

Both of thefe Inconvenienries are prevented by the pre- fent Structure of the Labyrinth and Cochlea, whofe Canals, by their winding, contain large Portions of the auditory Nerve, upon every Point of which the fmalleft Sound be- ing at once impreffed becomes audible 5 and by their Nar- rownefs, the Sounds are hindred from dilating : And the Imprcffions made upon the Nerves by the firli Dilatations are always the ftrongeft.

The Strength of the Impreffion in narrow Canals, is likewife increafed on Account of the Elafticity of the Sides of the bony Canal; which receiving the firfl and ftrongeft Impulfcs of the Air, do reverberate them more ftrongly upon the auditory Nerve.

It may be obferved, that tho' Air be the ufual Matter


 * " of Sounds j fo that a Bell rung in Vacuo is not heard

at all : Yet will moft other Bodies, properly difpofed,

do the like Office 5 only fome more faintly than others.

' See Medium.

Thus may a Sound be heard thro' Water, or even thro' Earth 5 of which we have various Inftances. See Sound.

Add, that tho' the Ear be the ordinary Organ of Hearing; yet Hagerup, a 'Danift Phyfician, maintains, that one may hear with the Teeth. — Thus, if one End of a Knife, or the like, be applied on a Harpfichol, and the other held between the Teeth; the Mufic thereof will be plainly heard, tho' the Ears be ever fo clofely ftopp'd. . — But this, perhaps, may as well be refcrr'd to the Senfe of Feeling. See Feeling.

Such as Want the Senfe at Hearing are faid to be Mute or tDeaf. See Deafness.

' The Senfe of Hearing, fays Cicero, is always open 5


 * for this we have Need of even when afleep. The

• Paffage to it is full of Turns, and Meanders; that

' little Vermine does endeavour to pafs 5 it mull {lick and « be bemircd in the Cerumen, or Ear-Wax, laid for that
 * nothing hurtful may enter or find its Way in. If any

• Purpofe near the Entrance.' %)e Nat. 'Deor. L. 2 Cap. 57.

Hearing, is particularly ufed in Civil and Judicial Concerns, for a Caufe, being brought before the Judge and Jury, and the Parties being heard as to the Merits thereof. See Cause, Judge, &c.

Such a Caufe was kept off eight Months, e're it was brought to a Hearing. — We are to have our Hearing the laft Day of the Term. See Issue.

The Hearing of Embaffadors at the Courts of Princes is ufually call'd Audience. See Audience and Em- bassador.

HEARSE, among Hunters, an Hind in the fecond Year of her Age. See Hunting.

HEART, Cor, in Anatomy, a mufculous Part, in the animal Body, fituate in the Thorax; wherein the Veins all terminate, and from which all the Arteries arife 5 and which, by its alternate Contraction and Dilatation, is the chief Inftrumcnt of the Circulation of the Blood, and the Principle of vital Acfion. See Artery, Vein, Blood, Life, Wc.

This noble Part is included in a Capfula, or Pouch, call'd the 'Pericardium; whofe Structure and Office will be explain'd under the Article Pericardium.

The Figure of the Heart is a Cone, or Pyramid re- verfed 5 the upper, and broader Part whereof, is called the ffafis; and the lower, the Cone, Apex, or 'Point; which is turn'd a little towards the left Side.

Its Magnitude is indeterminate, and different in feveral Subjects, according to their refpective Dimensions. ■ — ■ Its ordinary Length is about fix Inches, its Breadth at the Bafis four or five 5 and the whole Circumference fourteen. Its Place is in the middle of the T~horax, between the two Lobes of the Lungs 5 and it is faften'd to the Media- stinum and Pericardium, and fupportcd by the great

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HEA

Blood-Vefle Is, to which alone it is immediately conneSed - being, for the Convemency of its Motion, difeneased from any other Impediments. — It is cover'd with a thin Mem- brane, which, about the Bafis, is guarded with fat • and which is no other than the common Membrane of the Mufcles. See Membrane.

It has two great Cavities, call'd Ventricles, fomewhat unequal; the right being larger, capable of containing between two and three Ounces of Blood. — . They are di- vided by a flefhy Partition, confifting of the fame mufcular Fibres with the Parietes themfelves, and called the Septum- the Figure whereof is Concave towards the left Ventricle' and Convex towards the right. . — There is no immediate Communication between the Ventricles; but for the Blood to pafs out of one into the other, it muft fetch a round thro' the Lungs. See Septum, (3c,

The Parietes, or Sides of thefe Ventricles, are of a Thicknefs and Strength very unequal; the left much exceeding the right, becaufe of its Office, which is td force the Blood thro' all the Parts of the Body; whereas the right drives it through the Lungs onlv, and is therein greatly affifted by other Parts. — The right Ventricle, in Effect, feems only intended with a View to the Lungs; whence, in fuch Animals as have no Lungs, we only find one Ventricle, which is the left. See Lungs.

In the Ventricles are little Mufcles, called Columns Carney, or Lacertuli, derived from the Parieties, and connefted, by tendinous Extremities, to the Valves of the Heart hereafter mention'd. See Columnje.

The Ventricles are capped each with an Auricle, or little Mufcle, confifting, like the Ventricles themfelves, of a double Order of flefhy Fibres. See Auricle.

The Veflels, either arifing from, or terminating in the Heart, and its Auricles are two Arteries, viz. the Aorta, and the Pulmonary Artery, which have their Origin from the two Ventricles, viz. the Aorta from the left, and the Pulmonary from the right .- And two Veins, which ter- minate in the Auricles, viz. the Cava Vein in the tight, & and Pulmonary Vein in the-right. See Aorta, Cava, _v Pulmonary, c^c.

At the refpective Orifices of thefe Veffels are placed Valves. See Valve.

Particularly, at the Orifice of the Arteries, whhin each of them, are three femi-lunar Valves, or Membranes, of a femi- lunar Figure, which ciofe the Orifice of the Artery, and hinder the Relapfe of the Blood into the Heart at the Time of its Dilatation. < — ■ At the Mouth of the right Ventricle, jure at its Juncture with the Auricle, arc three others call'd. T'ricltfpides, from their three Points being faften'd by tendinous Fibres to the Columns Carnecs; fo that upon the Contraction, or Syftole of the Heart, they clofe the Orifice, and hinder the Blood from recurring into the great Vein. ■ — ■ The fame Office do the two Mitral Valves, at the Exit of the left Ventricle, flopping the Return of the Blood into the Pulmonary Vein. See SiiMi-Lunar, Tri- cuspid, and Mitral.

The Subftance of the Heart is entirely flefhy, or mufc culous. — ' The Antients, indeed, generally took it for a Parenchyma; but Hippocrates had a jufter Sentiment - and after him Steno and the Moderns", have evidently found it to confift of a continued Scries of proper mufculous Fibres varioufly contorted, or wound up, and ending at the Orifices of the ref'pecfive Ventricles, where they form their Tendons.

In diffecling the Heart, "after taking off the proper Membrane, there appear, on the outer Surface of the right Ventricles, fome {lender ftrait Fibres tending to and end- ing in the Bafis. — ■ Immediately under thefe is a double Order of fpiral Fibres, the exterior whereof afcend ob- liquely from the Septum to the Bafis, and form a Sort of Helix, or Cochlea : The interior take a contrary Courfe 5 winding obliquely from the right Side towards the loft, fo as to encompafs both Ventricles, and ending in the Bafis on the left Side, form likewife an Helix of an inverfe Order. ■ — ■ Under thefe appear the Fibres of the left Ventricle; and firft, a fpiral Series running to the leftj under which, as in the other Ventricle, lie another Order running the contrary Way, which not only extend to the outward Paries, but encompaffing the whole Ventricle, make the Septum more immediately appertain to, and be, a Part of, the left Ventricle. Some of them, inftead of terminating, as the reft do, in the Tendons of the Heart, run inwards, and form the Columns Carney; while others, reaching down to the Cone, are wound about it, and form the Circle called the Centre of the Heart. See Centre.

The Fibres of the Heart appear to be the fame with thofe of the other Mufcles; whence the Part now generally paffes for a real Mufcle; tho' fome think the Inference not over juft, inafmuch as the Aorta has the fame Claim to be reputed a Mufcle. See Mvscle and Aorta.

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