lize the
pressure on either side of the drum-head. A comparative study of the
ears of the vertebrata brings to light the fact that, as we descend in
the animal scale, the four ear ossicles are replaced by large bones
and cartilages connected with the jaw, and the drum and Eustachian
tube by a gill slit. We have, in fact, in the ear, as the student will
perceive in the sequel, an essentially aquatic auditory organ, added to
and patched up to fit the new needs of a life out of water.

Section 117. The impressions of smell are conducted through the first
nerve to the brain, and are first received by special hair-bearing cells
in the olfactory mucous membrane of the upper part of the nasal
passage. The sense of taste has a special nerve in the ninth, the
fibres of which terminate in special cells and cell aggregates in the
little papillae (velvet pile-like processes) that cover the tongue.

Section 118. At an early stage in development, the brain of a
mammal consists of a linear arrangement of three hollow vesicles
(Figure 5, Sheet VIII., 1, 2, and 3), which are the fore-, mid-, and
hind-brain respectively. The cavities in these in these vesicles are
continuous with a hollow running through the spinal cord. On the
dorsal side of the fore-brain is a structure to be dealt with more fully
later, the pineal gland (p.g.), while on its under surface is the
pituitary body (pt.).

Section 119. The lower figure of (5) shows, in a diagrammatic manner,
the derivation of the adult brain from this primitive state. From the
fore-brain vesicle, a hollow outgrowth on either side gives rises to the
(paired) cerebral hemisphere (c.h.), which is prolonged forward as the
olfactory lobe (o.l.). From the fore-brain the retina of the eye and the
optic nerve also originate as an, at first, hollow outgrowth (op.). The
roof of the mid-brain is also thickened, and bulges up to form two
pairs of thickenings, the corpora quadrigemina, (c.q.). The hind-brain
sends up in front a median outgrowth, which develops lateral wings,
the cerebellum (cbm.), behind which the remainder of the hind-brain is
called the medulla oblongata, and passes without any very definite
demarcation into the spinal cord.

Section 120. Figure 1 is a corresponding figure of the actual state of
affairs in the adult. The brain is seen in median vertical section. (ch.)
is the right cerebral hemisphere, an inflated vesicle, which, in the
mammal-- but not in our lower types--