168 
THE EAR AND HEARING.. 
nearly through its thickness, thus giving to it great flexibility. 
It is attached to and moved upon the head by three sets 
of muscles—anterior (attrahens), superior ( attollens '), and 
posterior (retr aliens ). It possesses also several intrinsic 
muscles, which in man are feeble and scarcely distinguishable, 
seeming to be merely rudimental of those more highly 
developed muscles found in the large ears of most quadru¬ 
peds, enabling them to alter the form and direction of the 
auricle. The divisions of the Pinna are (1) the Helix, or 
outer rim; (2) the Anti-lielix, parallel with it; (3) the 
Concha, or deep depression which focusses and reflects sound 
vibrations ; (4) the Notch, or lowest part of the concha ; (5) 
the Tragus, on the cheek side of the notch ; (6) the Anti 
Tragus; and (7) the Lobe, or soft pendulous part below the 
notch. The canal (Meatus Externus) opening to the Drum is 
about an inch to an inch and a-half long—shortest in child¬ 
hood, when the inner or bony portion of the meatus is 
wanting, being represented by a mere tympanic ring. 
Its direction is curved, and slightly tortuous, convex 
above, and oval in section. It is closed internally by the 
nearly circular membrane of the drum, which is inclined 
outwardly at an angle of about 45deg. with the floor of the 
canal. The canal is lined in its external part with fine hairs, 
and studded with ceruminous glands, most numerous about 
the centre. These glands secrete the Cerumen or ear wax, 
which serves partly for cleansing the passage, and partly 
perhaps, by its intensely bitter taste, to keep out insects. 
The Middle Ear or Tympanum is the irregular space 
between the tympanic membrane and the internal ear. It 
measures about Jin. from the membrana tympani to the 
vestibule, and about Jin. in its other two diameters. The 
tympanic membrane is fitted into a groove in a ring of bone, 
which is said to assist in conveying to the membrane sound 
vibrations communicated through the cranial bones. The 
membrane is thin and semi-transparent, but consists of three 
layers—an outer layer continuous with the cuticle of the 
external meatus—an inner layer continuous with the mucous 
membrane of the tympanum, and a middle layer of fibrous 
tissue on which its strength depends; very elastic and con¬ 
taining blood vessels and nerves. Its fibres are chiefly 
radial, but there appear to be a few circular ones near the 
outer edge. The roof of the tympanum communicates 
posteriorly with a number of air cavities (the mastoid cells 
in the mastoid process of the temporal bone). In the floor 
is the opening of the eustachian tube, a narrow canal about 
an inch and a half long, enlarged at both ends, especially at 
