THE CRANIAL NERVES AND SPECIAL SENSE ORGANS 
93 
The ear region may be reached by sawing horizontally through 
the head (or more conveniently the half head) just above the level 
of the internal acoustic meatus, the external acoustic meatus, 
and the pharyngeal orifice of the auditory (Eustachian) tube. 
The external ear consists of the auricle (usually removed in 
skinning the specimens) and the acoustic canal, which leads 
through the external acoustic meatus to the cavity of the middle 
ear, the cavum tympani, from which it is separated by the delicate 
membrane of the tympanum stretched across its inner end. By 
dissecting away the soft parts and carefully chipping off the bony 
roof of the acoustic canal and the cavum tympani, these cavities 
may be opened from above with the membrane of the tympanum 
in place between them. 
The cavum tympani communicates with the pharynx through 
the auditory or Eustachian tube \vhich may also be laid open from 
above. (There also open into the cavum tympani from below, the 
large cavities or “cells” of the mastoid bone, as may be demon¬ 
strated from a preparation of the human temporal bone sawn 
through the cavum tympani and the mastoid process.) 
The membrane of the tympanum bears attached to its inner 
surface, the first of a chain of three tiny bones, the auditory 
ossicles; this first ossicle is the malleus, and is in turn articulated 
with the incus, while the latter bears the stapes; the stapes has a 
ring-shaped portion which fits into the fenestra vestibuli, an open¬ 
ing in the bony wall of the vestibule of the internal ear. Thus 
sound vibrations which move the tympanic membrane are com¬ 
municated through this chain of bones to the perilymph and 
through this to the endolymph, and so stimulate the receptive 
organs which are in the lining of the membranous labyrinth. A 
second fenestra in the bony wall of the labyrinth, the fenestra 
of the cochlea, also opens into the cavum tympani beneath the 
fenestra vestibuli, and the membrane which covers it furnishes 
the amount of movability in the wall of the perilymph cavity 
necessary to permit the vibrations of the fluids. 
Two muscles may be seen in the cavum tympani, (i) the tensor 
tympani, which arises from the medial surface of the wall of the 
auditory tube and is inserted, by means of a tendon which is bent 
nearly at right angles, into the malleus, and (2) the stapedius 
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