in, within the bevelled border of the
sclerotic, to which it is firmly attached. The cornea is composed of
several different layers; its blood-vessels are so small that they
exclude the red particles altogether, and admit nothing but serum.

898. The CHOROID COAT is a vascular membrane, of a rich chocolate-brown
color upon its external surface, and of a deep black color within. It
is connected, externally, with the sclerotic, by an extremely fine
cellular tissue, and by the passage of nerves and vessels; internally,
it is in contact with the retina. The choroid membrane is composed of
three layers. It secretes upon its internal surface a dark substance,
called _pig-ment'um ni'grum_, which is of great importance in the function
of vision.

899. The IRIS is so called from its variety of color in different
persons. It forms a partition between the anterior and posterior
chambers of the eye, and is pierced by a circular opening, which is
called the _pu'pil_. It is composed of two layers. The radiating
fibres of the anterior layer converge from the circumference to the
centre. Through the action of these radiating fibres the pupil is
dilated. The circular fibres surround the pupil, and by their action
produce contraction of its area. The posterior layer is of a deep
purple tint, and is called _u-ve'a_, from its resemblance in color to
a ripe grape.

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How are this coat and the cornea united? 897. Describe the cornea.
898. What is the color of the external surface of the choroid coat? Of
the internal? How is it connected externally? How internally? What
does this membrane secrete upon its internal surface? 899. Describe
the iris. Of how many layers of fibres is the iris composed? What is
the function of the radiating fibres? Of the circular?

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900. The CILIARY PROCESSES consist of a number of triangular folds,
formed, apparently, by the plaiting of the internal layer of the
choroid coat. They are about sixty in number. Their external border is
continuous with the internal layer of the choroid coat. The central
border is free, and rests against the circumference of the crystalline
lens. These processes are covered by a layer of the pigmentum nigrum.

[Illustration: Fig. 138. A view of the anterior segment of a transverse
section of the globe of the eye, seen from within. 1, The divided edge of
the three coats--sclerotic, choroid, and retina. 2, The pupil. 3, The
iri