ent, and the nerve fibres are gathered
together, no visual impressions are recorded. If there is any doubt as
to the existence of a blind spot in the retinal picture, the proof is easy.
Let the reader shut his left eye, and regard these two asterisks, fixing
his gaze intently upon the left-hand one of them.

* *

At a distance of three or four inches from the paper, both spots will be
focussed on his retina, the left one in the centre of vision, and the
right one at some spot internal to this, and he will see them both
distinctly. Now, if he withdraws his head slowly, the right spot will of
course appear to approach the left, and at a distance of ten or twelve
inches it will, in its approach, pass over the blind spot and vanish, to
reappear as he continues to move his head away from the paper.
The function of nerve fibres is simply conduction, and the nature of the
impressions they convey is entirely determined by the nature of their
distal and proximal terminations.

Section 114. Certain small muscles in the orbit (eye-socket) move the
eye, and by their action contribute to our perception of the
relative position of objects. There is a leash of four muscles rising
from a spot behind the exit of the optic nerve from the cranium to the
upper, under, anterior, and posterior sides of the eyeball. These are
the superior, inferior, anterior, and posterior recti. Running from the
front of the orbit obliquely to the underside of the eyeball is the
inferior oblique muscle. Corresponding to it above is a superior oblique.
A lachrymal gland lies in the postero-inferior angle of the orbit, and a
Handerian gland in the corresponding position in front. In addition to
the upper and lower eyelids of the human subject, the rabbit has a
third, the nictitating lid, in the anterior corner of the eye.

Section 115. The ear (Sheet VII.) consists of an essential organ of
hearing, and of certain superadded parts. The essential part is called
the internal ear, and is represented in all the true vertebrata (i.e.,
excluding the lancelet and its allies). In the lower forms it is a hollow
membranous structure, embedded in a mass of cartilage, the otic
capsule; in the mammal the latter is entirely ossified, to form the
periotic bone. The internal ear consists of a central sac, from which
three semicircular canals spring. The planes of the three canals are
mutually at right angles; two are vertical, the anterior and posterior
(p.v.c.) v