(folio 11 recto) The socket of the eye is not over-depressed, for it
has to receive the images (_spetie_) of visible things. Nor does it
project greatly, lest it should be liable to injury from exterior
violence. For the eyes of man being very soft and susceptible, nature
provided eyebrows as a shield above, and eyelids as protectors in
front, and made moreover the projections of the maxillae and the nose,
so that the eyes should be guarded on every side. So great was the
solicitude of nature for these members.
Seven are the tunics of the eye and three its humours. Three front
coatings join with three coatings at the back like six shields, the
edges of every pair joining each to each, the outer being larger
and containing the others. The seventh tunic is largest of all, and
encloses the whole eye, and therefore it is called _conjunctiva_
because it joins and surrounds the whole eye except the place where the
pupil is, and that small part [is covered] by the cornea. Now this
first tunic where it covers the outside part is seen to be white.
The second tunic in its front part is called _cornea_ because it
resembles horn in its substance and colour; and this covering is
transparent, so that the images of visible things may penetrate through
it. And it is also solid and large and composed of four membranes, so
that being near external things it should not receive hurt. With this
[corneal tunic] is united posteriorly another tunic [the third] called
_sclerotic_, i.e. hard. These two coverings have their origin in the
membrane about the brain, that is in the dura mater, just as the first
tunic arises from the membrane over the skull, called _almochatim_.
The fourth tunic as to its front part is called _uvea_ [because] it is
like a seed of a black grape, and in its midst is a hole called the
pupil. Nature made this tunic opaque so that the visual spirit should
be conserved and not dissipated by the light outside. Moreover nature
made the opening in the tunic that the image might penetrate freely;
while it is narrow, so that the visual spirit should be concentrated.
Thus when the said pupil, or rather hole, dilates more than usual,
either naturally or accidentally, the sight becomes imperfect. [The
uveal tunic] joins posteriorly the fifth tunic, called _secundina_
because it is made like the after-birth, i.e. the membrane in which the
child is enveloped in its mother’s womb, and it arises from the pia
mater.
The sixth coating in front is called _arachnoid_ because it is formed
after the manner of a spider’s web, and posteriorly it joins the
seventh coating, called _retina_, because it is made like a net.
Between the uvea and the arachnoid anteriorly there is a humour called
_albugineus_, like the white of an egg, to moisten the eye and to
preserve the convexity of the cornea. In a dead man this humour dries
up, and the cornea falls and is flattened, and then the vulgar say that
there appears a curtain before the eyes which is an infallible sign of
death. Also this humour holds the pupil open; therefore when it dries
up the pupil contracts.
Between the two last tunics, i.e. the arachnoid and the retina, which
have their origin from the optic nerve, there are two humours. These
are the _vitreous_ humour, so called from its likeness to liquified
glass, and the _crystalline_ humour, from its likeness to a crystal.
This is also called the _grandid_, because it is like a hailstone;
and it is somewhat hard and round, but flattened anteriorly where
it receives the images of visible things, and posteriorly pyramidal
shape and pointed. And here is completed the act of seeing. In the
posterior part it is surrounded by the vitreous humours by which it is
nourished. The crystalline humour is convex anteriorly and the vitreous
posteriorly. And the optic nerves come to the eyes and convey the
images seen by the eyes to [the seat of] common sensation and to the
other internal faculties. [Folio 12 verso, line 7.]
* * * * *
[Illustration: Fig. 19. THE ANATOMY OF THE EYE
From G. Reisch, _Margarita philosophiae_, Leipzig,? 1503. Showing the
seven tunics and three humours of the eye according to the doctrines of
Renaissance anatomists.[196]]
A great deal of attention was paid by the Arabians to the diseases and
the structure of the eye, and the essentials of Manfredi’s description
are to be found in Rhazes, Hunain ben Ishak, and Haly Abbas. The
tradition presented by these writers passed early into Western
science, and is reproduced, for example, in the works of Constantine
Africanus and in the well-known anatomy to which the name of Richardus
Anglicus (Richard of Wendover) has become attached[197] (cp. Fig.
19). Avicenna’s description of the eye is somewhat different, and
gave rise to the tradition reproduced in the works of John of Peckham
and of Roger Bacon (Plate XXXVIII _a_), and it influenced the views
of Leonardo and even perhaps of Vesalius (Fig. 20). The views on the
anatomy of the eye expressed by Rhazes, Hunain ben Ishak, and Haly
Abbas were, on the whole, more widely accepted than those of Avicenna.
[Illustration: Fig. 20.