n6 THE GROUNDWORK OF SCIENCE

with those which go to the lungs, stomach, and heart.

Perhaps the most important, for our purpose, of all the

structures which make up our bodily frame, are those organs

by the aid of which, in unison with the brain, we are enabled

to have sensations of different kinds.

The organ of sight consists essentially of an extremely

delicate membrane, the retina, wherein are a multitude of

minute bodies called rods and cones placed side by side, and

lining the rear of the eyeball. The retina is an expansion of

the optic nerve (or nerve of sight), through which it is directly

continuous with the substance of the brain itself.

The eyeball is bounded by a tough spherical case, and

contains within it three transparent media, of different

densities, while it is itself transparent anteriorly. It also

contains a mechanism to facilitate vision at different distances,

and its transparent media produce a picture (though an in-

verted picture) of what is opposite the eye, on the posterior

part of the internal lining of the eyeball.

As each eye forms an image of what is opposite it, the two

pictures simultaneously formed in the two eyes slightly differ

from each other. They, of course, must do so, since each

looks out on the world from a different point of view.

The essential organ of hearing in man (and also in back-

boned animals) consists of most delicate nervous fibres, which

are distributed over a small, complexly shaped membranous

bag containing fluid, and itself surrounded by another fluid,

which is enclosed in a cavity (corresponding in shape to the

bag it encloses) in the densest bone of the skull, some

distance within the opening on the surface of the side of the

head, surrounded by that conspicuous projection commonly

spoken of as "the ear." The nerve of hearing passes outwards

from the brain, traverses a canal through the dense bone

just referred to, which canal gives it entrance into the cavity,

wherein lies the membranous structure before mentioned,

with those which go to the lungs, stomach, and heart.

Perhaps the most important, for our purpose, of all the

structures which make up our bodily frame, are those organs

by the aid of which, in unison with the brain, we are enabled

to have sensations of different kinds.

The organ of sight consists essentially of an extremely

delicate membrane, the retina, wherein are a multitude of

minute bodies called rods and cones placed side by side, and

lining the rear of the eyeball. The retina is an expansion of

the optic nerve (or nerve of sight), through which it is directly

continuous with the substance of the brain itself.

The eyeball is bounded by a tough spherical case, and

contains within it three transparent media, of different

densities, while it is itself transparent anteriorly. It also

contains a mechanism to facilitate vision at different distances,

and its transparent media produce a picture (though an in-

verted picture) of what is opposite the eye, on the posterior

part of the internal lining of the eyeball.

As each eye forms an image of what is opposite it, the two

pictures simultaneously formed in the two eyes slightly differ

from each other. They, of course, must do so, since each

looks out on the world from a different point of view.

The essential organ of hearing in man (and also in back-

boned animals) consists of most delicate nervous fibres, which

are distributed over a small, complexly shaped membranous

bag containing fluid, and itself surrounded by another fluid,

which is enclosed in a cavity (corresponding in shape to the

bag it encloses) in the densest bone of the skull, some

distance within the opening on the surface of the side of the

head, surrounded by that conspicuous projection commonly

spoken of as "the ear." The nerve of hearing passes outwards

from the brain, traverses a canal through the dense bone

just referred to, which canal gives it entrance into the cavity,

wherein lies the membranous structure before mentioned,