830

Advantages in the structure of the eye in certain animals.

a b n is the membrane which closes the eye from below, upwards, with an opaque film, c n b encloses the eye in front and behind with a transparent membrane.

It closes from below, upwards, because it [the eye] comes downwards.

When the eye of a bird closes with its two lids, the first to close is the nictitating membrane which closes from the lacrymal duct over to the outer corner of the eye; and the outer lid closes from below upwards, and these two intersecting motions begin first from the lacrymatory duct, because we have already seen that in front and below birds are protected and use only the upper portion of the eye from fear of birds of prey which come down from above and behind; and they uncover first the membrane from the outer corner, because if the enemy comes from behind, they have the power of escaping to the front; and again the muscle called the nictitating membrane is transparent, because, if the eye had not such a screen, they could not keep it open against the wind which strikes against the eye in the rush of their rapid flight. And the pupil of the eye dilates and contracts as it sees a less or greater light, that is to say intense brilliancy.

Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.

Notebooks of Leonoardo da Vinci
XIV: Anatomy, Zoology and Physiology.
. . .
810,
811
The divisions of the head.
812,
813
Physiological problems.
814,
815
The divisions of the animal kingdom.
816,
817
Miscellaneous notes on the study of Zoology.
818,
819,
820,
821
muscles.
822,
823,
824,
825,
826
Comparative study of the organs of sense in men and animals.
827
Advantages in the structure of the eye in certain animals.
828,
829,
830,
831
Remarks on the organs of speech.
832,
833
On the conditions of sight.
834,
835
The seat of the common sense.
836
On the origin of the soul.
837
On the relations of the soul to the organs of sense.
838
On involuntary muscular action.
839
Miscellaneous physiological observations.
840,
841,
842
The laws of nutrition and the support of life.
843,
844,
845,
846,
847
On the circulation of the blood.
848,
849,
850
. . .