THIRD SECTION.

Of the Organs of the Head, Drawn from Nature or from Casts, in Different Positions.

I. The Eyes.

We have seen, plate 2, fig. 1 (page 10), the construction of the interior of the eye, and how light brings the image of objects to be painted at the back of the eye on the retina. Now, we will focus on the external construction of this organ and its position.

Position of the Eyes.

The eyes are placed below the forehead in two bony cavities called orbits. The eyeball is suspended in the orbit by four straight muscles and two oblique ones; these muscles contract to direct the eyes towards the objects that attract our gaze. It is the eyeball that turns in the orbit according to the will to look right or left, and not the edges. The eyeball is more or less recessed in the orbit: the orbicular muscle covers a large part of it. The upper eyelid, when lowered, entirely covers what we should not see of the eye. The eyelids are two mobile, flexible membranes so thin that the eye distinguishes between day and night through their thickness. The main use of the eyelids is to protect the eyes from the constant action of light. Curved over the eyeball, they take the shape of the part they cover. The edge of the eyelids is fringed with lashes: those of the upper lid are longer and more numerous than those of the lower. The edge of the eyelids has a thickness where the lashes are attached. In what looks down, what looks at the horizon, it is the thickness of the lower eyelid's edge that is lit. In this position, the thickness of the upper eyelid’s edge is entirely withdrawn from the light. If, on the contrary, the eye looks upwards, it is the thickness of the upper eyelid that is lit, and the thickness of the lower eyelid that is in shadow.

The parts of the eye where the eyelids join are called angles or more commonly corners of the eyes. What is near the nose, i.e., the internal angle, presents a rounded notch, where the edges unite externally with the edges and thickness of the eyelids: the edges of this angle are tortuous and fringed: the protruding reddish part of the lacrimal caruncle is what appears. We call this angle indifferently caruncle or internal angle. The external angle is the one towards the ear.

Proportions of the Eyes.

The opening of the eyelids, from one angle to the other, reveals almost the entire diameter of the eyeball. The pupil or transparent cornea usually occupies the large third of the eyeball's diameter. The pupil or eye hole occupies a third of the pupil; see plate 17, fig. 1. When an eye is seen from the front, the eyeball and therefore its pupil are, in relation to the observer, in the position of the one represented here, fig. 2, by a b c, and the pupil is represented here by the pupil a as seen from the front.

We have supposed here, fig. 9, the eyeball seen in different positions. The eyeball being spherical, whatever position it is placed in, it will always have the figure of a sphere: but the pupil, which is on the surface, undergoes foreshortening. Thus, if we assume the eye seen from the front, the pupil a is seen from the front.

Translation Notes:

- "Orbicular muscle" refers to the muscles around the eyes which help in eyelid movement.
- "Lacrymal caruncle" is a small reddish body at the inner corner of the eye.