Section 116. The eighth nerve runs from the brain case (Cr.), into the periotic bone, and is distributed to the several portions of this labyrinth. In an ordinary fish this internal ear is the sole auditory organ we should find; the sound-waves would travel through the water to the elastic cranium and so reach and affect the nerves. But in all air-frequenting animals this original plan of an ear has to be added to, to fit it to the much fainter sound vibrations of the compressible and far less elastic air. A "receiving apparatus" is needed, and is supplied by the ear-drum, middle ear, or tympanic cavity (T.). In the mammal there is also a collecting ear trumpet (the ear commonly so-called), the external ear, and external auditory meatus (e.a.m.). A tightly stretched membrane, the tympanic membrane, separates this from the drum. A chain of small bones, the malleus (m.), the incus (i.), the os orbiculare (o.or.), a very small bone, and a stirrup-shaped stapes, swing across the tympanum, from the tympanic membrane to the internal ear. At two points the bony investment of this last is incomplete-- at the fenestra rotunda (f.r.), and at the fenestra ovalis, (f.o.), into which latter the end of the stapes fits, and so communicates the sound vibrations of the tympanic membrane to the endolymph. A passage, the Eustachian tube, communicates between the tympanic cavity and the pharynx (Ph.), and serves to equalize 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.).