Outlines of Creation

by Elisha Noyce

FIG. 85.—SECTION OF HUMAN EYE.

A. Sclerotic.	D. Vitreous humour.	G. Crystalline lens.
B. Choroid.	E. Aqueous humour.	H. Pupil.
C. Retina.	F. Cornea.	I. Optic nerve.

The eye consists of various humours of different densities enclosed in membranes (fig. 85); the human eye is spherical for the posterior four-fifths of its circumference, but the front part projects a little, being formed of part of a smaller sphere. The globe of the eye, all but this front part, is enclosed in a firm fibrous coat called the "sclerotic," within which is a membrane full of blood-vessels called the "choroid," this on its inner side is covered with a black substance called the "black pigment," and within this (lining the ball) is the nervous coat or retina, it is the optic nerve so expanded as to form a lining to the membranes of the ball of the eye for the greater part of its extent. The ball of the eye is filled with a transparent fluid, somewhat glutinous, called the "vitreous humour;" stretched across the front part of the ball of the eye is a membrane having a round hole in its centre, called the "iris," coated (like the choroid) with black pigment on its back surface; this membrane (the iris) is the coloured part of the eye, and its transparency or opacity determines the colour of the eye. The pupil is the hole in its centre, it looks black because the black pigment which covers the inside of the ball of the eye (for the retina is transparent) is seen through it; behind the iris and in front of the vitreous humour, is the crystalline lens, it is just the shape of a very thick lens, but is more convex behind than in front, it is quite transparent, and of a density almost approaching to solidity. The front fifth of the ball of the eye is covered in by a dense transparent substance called the cornea, of the form of a watch-glass and quite transparent, between this and the crystalline lens there is a small space called the anterior chamber, filled with a clear watery fluid called the aqueous humour. The eye is moved by six muscles, four straight and two oblique, these turn it in every direction with the greatest accuracy; the front of the eye is defended by the eyelids, which by winking and being moistened by the tears, keep the eye always moist and free from dust.

The eyes of insects and several other classes of animals are called compound eyes, and the image they produce must resemble objects depicted in mosaic, or made up of small spots of distinct colours; they consist of hundreds of small tubes, radiating and forming part of a sphere on each side of the head, each of these tubes admits the light from objects at which they point. Some of the animals still lower in the scale have simple spots of transparent membrane with an expansion of optic nerve beneath them; in such creatures the power of vision is probably limited to distinguishing light from darkness.

FIG. 86.—SECTION OF THE HUMAN EAR.
a, external opening; b, tympanum; c, small bones;
d, Eustachian tube; e, cochlea; f, semicircular canals.

Hearing.—Sound is conveyed to the mind by the vibration of air, and this vibration is communicated to a membrane within the ear, called the "tympanum;" this is in connection (by means of three minute bones) with the expanded auditory nerve, which ramifies within the "vestibule," which is filled with a fluid, so that the vibrations of the air are thus communicated to the nerve of hearing (fig. 86).

Smell.—The interior of the nose is lined by a fine moist membrane, on which the olfactory nerve is distributed. The cause of odour is quite unknown; though, as most volatile substances are odorous, it is supposed to be some volatile matter which comes in contact with the olfactory nerve, or nerve of smelling. The quantity given off in some instances must be exceedingly small, for one grain of musk placed on a piece of glass will give out scent for months, and at the end of that time weigh still one grain. Some animals, as Dogs and Birds of prey, have this sense so acute that it serves them instead of almost every other; the Vulture will scent carrion miles away, and the Dog will track his master by the power of scent.

Taste.—The sense of taste resides chiefly in the tongue, but to some extent in the palate or roof of the mouth; it appears to be a modified sense of smell, combined with common sensation or touch, and is much diminished by loss of smell.