An entire body of philosophy according to the principles of the famous Renate Des Cartes in three books, (I) the institution ... (II) the history of nature ... (III) a dissertation of the want of sense and knowledge in brute animals ... / written originally in Latin by the learned Anthony Le Grand ; now carefully translated from the last corrections, alterations, and large additions of the author, never yet published ... by Richard Blome.

CHAP. XVI. Of the Eye.

I. What the Eye is. FOrasmuch as the Eye, is the Organ of Sight, and that within the Recess or Hollow there∣of, the Images of the several Objects are repre∣sented, we shall never be able throughly to under∣stand how Vision or Sight is effected, without describing the disposition and structure of its parts. The Eye, therefore, is the outward Organ of Sight, through the Transparent parts whereof the Rays of Light pass, till they arrive at the Net-work-coat or Membran, and there variously moving the small Capillaments of the Nerves, suitable to the variety of Objects whence they proceed, do repre∣sent or pourtray the Image of the Object. The In∣ward Organ of this Sense are the Optick Nerves, which reach from the Net-work-like-membran to the Brain, and receiving the motion from the said Membran, convey it to the common Seat of the Senses.

II. Of the Fi∣gure of the Eye. And to the end we may more exactly appre∣hend all these particulars, suppose we an Eye cut through in the midst, but yet with that Art, as that all the several Humors contained therein, keep the same place they had before, without any part of them being spilt; and then it would ap∣pear to be of the same structure, as it is represent∣ed in this Scheme, wherein DB, CBD, set forth * 1.1its round Figure, that marked BCB, being the forepart of it, and BAB, the other part, which is enclosed within the Bone of the Head.

III. What the Horny Coat of the Eye is. BCB, is a hard and thick Membran, which the Physicians call the Horny Coat of the Eye, being, as it were the Vessel and Receptacle to contain all the other parts of the Eye; and this part is trans∣parent, and more prominent or convex than the Remainder, to which the Rainbow, so called, be∣longs, which almost is of different colours in all Men. It sticks out forwards, because if it were more flat, the Beams which slantingly touch its surface, would scarcely ever reach the Bottom of the Eye, through the entrance of the Apple there∣of; so that we should only perceive a very small part of the Hemisphere at one cast of the Eye.

IV. The Uveo••▪ or Grape-like Coat, and the Apple of the Eye. DEF, is a thinner Membran, stretched out like a Hanging or Tapistry, and is called the Tu∣nica Uvea or Grape-like Coat, for that it is per∣forated like a Grape when the Stele is pluckt out of it. In this Membran or Coat is a small hole, the Apple of the Eye, in the midst of the Rainbow, marked out by the Letters FF, called in Latin Pupilla, because in it a Baby or little Image is re∣presented by the Rays that are reflected from the surface of the Eye, to every one that looks upon the Eye of another. The Apple of the Eye, in a Man, appears Black, because that part of the Coroides, or the Grape-like Coat, which answers to it, is of that colour. The contexture of Fibres rang'd in a circle, and which outwardly is placed about the hole of the Apple of the Eye, being diversified with various colours, is called the Rain∣bow.

V. The Lig••∣ments of the Eye-lids. NN, are many black Filaments, called Proces∣sus Ciliares, which do every way surround, and gird in a certain soft and transparent Body, call∣ed the Crystalline Humour, and keep it suspend∣ed in the midst of them.

VI. The Watry Humour. The space contained betwixt EKE, is filled with another transparent Humour, which is there∣fore called the Watry Humour, because in all re∣spects it is like Water. This Humour gives the Round Figure to the Eye, refracts the admitted Beams, and in this disposition imparts them to the Crystalline Humour. The Eye continually re∣ceives of this humour by some particular Vessels which are in the Sclerotica, which is a part of the Horny Coat or Membran, and which are inserted into it, near to the Apple of the Eye, whence

it is that when the Horny membran is pierced, and the Watry Humour by this means is spilt and let out, the said loss may be repair'd in the time of a few Hours.

VII. The Cry∣stallin Humour. The Mid-part L, is a certain transparent Sub∣stance, rather of the figure of a Lentil, than Sphe∣rical; for the greatest part of it is cover'd by the Rainbow, and the Fore-part of the greater Round, is less than the other, and the Hind-part of the lesser Round, is the biggest. It is because of its Compactness and some appearing Hardness, called the Icy Humour, but more commonly the Crystal∣lin. This Humour is white, like Starch, and is of the consistence of Wax, which melts, and may be comprest, but cannot be spilt.

VIII. The Glassy Humour. The remaining Hollow of the Eye GMI, is taken up by a whitish Humour, more transparent than the Crystallin or Watry, and of a mean con∣sistence betwixt them both; because it can more easily be contracted and displayed; and yet it is not so liquid, or thin and flowing, as the Watry Humour. It is commonly called the Albumineous and Glassy Humour, because it hath the consistence and colour of the White of an Egg, and is trans∣parent like Glass. This Humour rests upon the Network-like Membran, and contains in it self the Crystallin; it is enclosed in a very thin Membran, which keeps it from spilling.

IX. The Cry∣stallin Hu∣mour caus∣eth much the same Refraction of the Rays, as Glass doth. Experience informs us, that the Crystallin Hu∣mour causeth much the same Refraction, as Glass and Crystal doth; and that the two other Hu∣mours produce somewhat a less Refraction, and much about the same as Common Water: So that the Rays of Light pass more easily through the Crystallin Humour, than through the two other Humours; and yet more easily through these, than through the Air.

X. The Optick Nerve. HZ, is the Optick Nerve, which takes its rise from the Hinder-part of the Brain, not far from the beginning of the Back-bone, the Capillaments whereof GHI, being disperst throughout the whole Space ABH, do cover the whole bottom of the Eye, constituting a sort of a most fine Net, which from its Expansion is called Retiformis, or Net-work like, and Retina by Physicians. Now these Capillaments, by means of the Convex figure of the Eye, and of the Crystallin Humour, do collect the Beams, and communicate the motion they have received to the Brain, and present it to the Soul that resides there.

XI. Why the Surface of the Coats of the Eye is black. My design is not to reckon up here all the Coats that enclose the Eye, seeing that the knowledge of them doth not conduce to the understanding of the Sense of Seeing; and therefore shall only take notice, that the surface of these Coats is wholly obscure and black in those Parts that face the bot∣tom of the Eye; and this, to the end that the Rays which fall upon the Net-like Coat GHI, and from thence are are reflected to the Grape-like Coat, might be extinguish'd by that Blackness, lest being again reflected to the Retina, or Net-like Coat, they should disturb and confound the Sight.

XII. The six Muscles of the Eye. OO, are six Muscles, outwardly fastned to the Eye, by the help whereof it can readily move it self every way. Of these Muscles 4 are called Direct, whereof the first lifts the Eye upwards, the second turns it downwards, the third draws it towards the Nose, and the fourth withdraws it to the opposite part from the Nose. The other 2 are called Oblique or Slanting, because they en∣compass and surround the Eye; for with the one of them the Eye is enabled to give a slanting or oblique Cast, and with the other it is rolled round.

XIII. How the Right or Direct Muscles serve to move the Eye. When the Right Muscle, which is above the Eye, is fill'd with Animal Spirits, the Eye looks up; and the 3 other Muscles being fill'd in like manner by turns, assist it sometimes to look down∣wards, and sometimes to turn it to the Right or Left. Besides, it is evident by the situation of these Muscles, that when all of them are shortned, they at the same time alter the figure of the Eye, by making it more flat than it was be∣fore.

XIV. Why the Apple of the Eye is of a Con∣vex figure. That part of the Eye, which is marked BCB, is of a Convex figure, to the end that the Rays proceeding from the Objects, which of themselves have not force enough to enter the Apple of the Eye FF, might be united by a various Refraction, and by this means might have force enough strongly to move the Hairy-strings of the Optick Nerve HZ. For the Refraction which is made in the Crystallin Humour L, procures strength and di∣stinction to the Sight.

XV. The Apple of the Eye can be contracted and dila∣ted. Besides, this is to be observed, that the Apple of the Eye is liable to Contraction and Dilatation, according as the Objects, to which the Eye is di∣rected, are nearer or farther off; or more or less enlightned; or according as the Beholder doth more intently or carelesly view the Object. For this Coat hath the power to dilate or narrow it self like a Muscle, and by this means to enlarge or contract the Apple of the Eye, viz. by stinting the entring of the Rays, and by causing more or less to pass through it.

XVI. The Motion of the Apple of the Eye is voluntary. Wherefore this motion may be called Voluntary, tho' for the most part it happen without our At∣tention; for it doth nevertheless depend on our Will, or on those motions which do accompany the Will or desire of throughly beholding any Objects. In like manner as the motion of the Lips and Tongue, conducing to the formation of Voices, is called Voluntary; because it is consequent to our intention of Speaking, tho' we do not mind, yea, and are ignorant also, what kind of motion every Letter requires.

Notes

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  Figure 68.