A Treatise on the Eye/Book 2/Chapter 2
CHAP. II.
Of the Connection of the Eyes.
Sect.
Pag.
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| 1. | What chiefly connects the Eyes to their Orbits, is the Membrana Conjunctiva or Adnata, | 60 |
| 2. | This Membrane shortly described, | 60 |
| 3. | Is covered with another Membrane from the Eye-lids, | 61 |
| 4. | They are both of a White Colour, and why, | 61 |
| 5. | They swell excessively in Ophthalmias, but without Danger, unless the inward Parts of the Eye be also affected, | 62 |
| 6. | They have a transparent Aponeurose covering them, which extends over the whole Cornea, | 63 |
| 7. | The Tunica Tendinea described, | 64 |
CHAP. II.
Of the Connection of the Eyes.
Sect. 1. Our Eyes Orbits, being placed in the are kept in that Situation by their Muscles, the optic Nerves, and the Fat lining the Orbits; to all which our Eyes are attached, But that which chiefly connects the Eyes to their Orbits, is a Membrane called Conjunctiva or Adnata. This therefore I shall here only consider, omitting the others, which being less concerned in this Matter, fall more naturally to be considered in another Place.
§ 2. The Conjunctiva takes its Origin from the Periosteum, all round the Edge of the Orbit, and from thence is extended over the whole Fore-part of the Globe, till its Termination in the Edge of the Sclerotica adjoining to the Cornea, where of consequence it forms a large Hole for the transparent Cornea, thro' which the Iris and Pupil are seen. From its Situation, it is called Adnata, and from its. Office, Conjunctiva.
§ 3. This Membrane is nothing but a Continuation of the Periosteum lining the Orbits internally. It is covered externally with another Membrane; for, as is known to Anatomists, the internal Membrane of the Eye-lids at the Edge of the Orbit, folds back again over the outward Face of the Eye above the Conjunctiva, to which it adheres, and with which it terminates at the Edge of the Sclerotica, where the Sclerotica joins the Cornea.
These two Membranes, because of their closs Union, appear to be only one, and are generally described as such, under the Name of Membrana albuginea, so called, because they form the White of the Eye; tho' in fact they are distinct Membranes, the one a Continuation of the Periosteum, lining the Orbits internally, and the other of the inner Membrane of the Eye-lids, which, notwithstanding their strict Union, may easily be separated, and have their distinct Origins clearly demonstrated.
§ 4. If it should here be asked, For what Ends these Tunicles are made always of a shining white Colour? It may be sufficient to answer: first, That such a Colour adds greatly to the Beauty of the Eye and Countenance; and, secondly, That, by surrounding the Pupil and Iris, this Colour, by its Opposition, renders them more bright and conspicuous, by which Means we are better enabled to judge of the Direction of the Eyes, and when any body looks at us; which in many cases is of considerable Use to us, and which could not be so easily known, were our Eyes all over of the same Colour; as has been well observed by that learned Mathematician, and great Philosopher, Joannes Keplerus. Hence, in Dogs, and some other Creatures that have the Fore-part of their Eyes mostly of the same Colour, it is extremely difficult for us to judge towards what Object their Eyes are turned, especially if they be at any considerable Distance from us.
§ 5. These Membranes, especially the external one, coming from the Eye-lids, are so full of Blood-vessels, and so laxly extended over the Eye, that in violent Ophthalmia's, the White of the Eye is sometimes swelled so excessively, as to cover all the Cornea; which I here take Notice of, because it is ready not only to surprise, but to impose on the unwary or unexperienced Oculist, as if it were an incurable Excrescence of the Cornea itself; whereas the Danger of such Ophthalmia's lies not so much in the swelling of this Membrane, as in some of the other Parts of the Eye, which, when affected at the same time, makes the Disease more or less dangerous, according as this or the other Part is more or less affected; all which may be known by the Symptoms, and is of the greatest Use to be observed in Practice; as might easily be shown in a Variety of Cases, were this a proper Place for such Inquiries.
§ 6. Besides these two Membranes, the Fore-part of the Globe is covered all over externally with a very thin transparent Aponeurose or Surpeau, which not only covers, the Membrane which it has from the Eye-lids, but is also extended beyond it over the Cornea itself. The Phlyctene, which are small transparent Vessicles full of clear Water, and which are frequently observed upon the Surface of the Cornea itself, as well as upon the White of the Eye, and even sometimes have their Centre in some Part of that Circle of the Cornea where it joins the Sclerotis, and by that Means occupy at the fame time, both a Part of the White of the Eye, and a Part of the Cornea, are, amongst other Things that might be advanced, a convincing Proof of the Existence of this Cuticle or Surpeau, and of its Extention over the whole Cornea.
This Cuticle seems to me to be a Production of the Epidermis, or Scarf-skin, which at the Edge of the Eye-lids folds inwards, so as to cover the Inside of the Eye-lids, and at the Edge of the Orbit folds back again over the whole outward Face of the Eye. The Exuviæ of Serpents, which are observed to be one continued Membrane that covered their whole Bodies, their Eyes, as well as the rest of their Body, is, amongst other Things that might be mentioned, a pretty strong Proof, not only[errata 1] of the Extention of this Cuticle over the whole Cornea, but also of its being a Production of the Epidermis.
§ 7. There is yet another Tunicle covering the Fore-part of the Globe, and connecting it to the Orbit, which had escaped the Observation of most Anatomists, till Columbus took Notice of it, and which therefore, it is probable, had been considered as a Part or Pellicle of the Conjunctiva, from which nevertheless it is quite distinct.
This Tunicle arises from the Tendons of the four streight Muscles of the Eye, and expanding itself over the Fore-part of the Globe, betwixt it and the Conjunctiva, to both which it adheres, it terminates at the Edge of the Sclerotis, where it forms the Cornea. Columbus assumes the Honour of being the first Discoverer of this Tunicle, to which he has given no Name. Hence it is frequently called Tunica Innominata Columbi, tho' unjustly, because it was known to Galen, as appears from the 2d and 8th Chapters of his tenth Book De usu partium. Others therefore, with better Reason, call it Tunica Tendinea, because formed of the Tendons of the four streight Muscles, of which it is an Extension or Aponeurose. Fabricius ab Aquapendente is of opinion, that the Conjunctiva has its shining Whiteness from this tendinous Coat. But in this he has been refuted by Plempius, who having separated these Coats, found the Conjunctiva of the same shining white Colour it had before Separation, and which being opake, can receive no Change of Colour from this tendinous Coat lying behind it.
It is by these Membranes chiefly that our Eyes are connected to their Orbits; and being soft, flexible and yielding, they do it in such a manner as not in the least to impede their necessary Motions.