THE exact nature of the communication of sound to the inner ear by bone conduction is somewhat obscure although the experimental work of Bekesy (1932), Barany (1938) and others offers suggestions of possible explanations of this means of stimulation. The relationship of the "occlusion" effect as observed when the meatus is plugged externally and internally and the effect of restriction of the conductive mechanism is discussed. It is suggested that in normal subjects and patients with perceptive deafness hearing by bone-conductor stimulation occurs virtually by means of the normal air-conduction mechanism due to the inertia of the free ossicular chain. In conductive deafness stimulation may be by direct excitation of the bony structure of the cochlea with the inertia of the cochlear fluid operative if the round window is unobstructed. In advanced stages of otosclerosis the obstruction of both windows would be expected to indicate an apparent perceptive element, i.e. a lowered bone-conduction audiogram. An account is given of experiments relating to the communication of sound from one ear to the other. When sound from a telephone receiver is applied to one ear the level of sound communicated to the other ear known as "cross-hearing" depends on the mode of application and the type of receiver. For a bone-conduction receiver applied to the mastoid there is in general very little difference in the level of sound reaching the two ears over the frequency range 250 to 2000 c/s. In the case of sound applied by an air-conduction receiver the communication of sound by "cross-hearing" suffers a reduction of 45 to 50 db. for an external receiver and 75 to 80 db. for an insert receiver. The importance of masking when making bone-conduction tests is emphasized and it is suggested that the application of the masking noise is accomplished most efficiently by means of an insert form of receiver. The usefulness of bone-conduction hearing aids and the conditions under which their application is superior or preferable to air-conduction types is discussed. Now let us consider the state of motion of the head when under the influence of a source of alter-Nov.-OTOL. AND LARYNG. 1

The pitch of a pure tone depends basically on its frequency, but it can sometimes be modified by the intensity of the tone. Hitherto, in work on the modifying influence of intensity (e.g. Stevens, 1935;Snow, 1936;Morgan, Garner & Galambos, 1951) the subjects have been chosen for their normality of hearing. It is reported by Morgan et al. (1951) that it is typical for such subjects to find pitch almost independent of intensity. In the present work, the effect of intensity on pitch for subjects with slight hearing defects has been examined.The intensity-dependence of pitch has been studied as a function of the magnitude ofthe hearing defect. This introduces a variable which is independent of any experiments on normal ears. APPARATUS AND TECHNIQUE Apparatu8For the air-conduction audiogram, an Amplivox model 61 audiometer was used.A block diagram of the apparatus used for the main series of tests is shown in Fig. 1. Two beat-frequency oscillators were used, B.S.R. type L.O. 50A, slightly modified to reduce noise. The output transformers were matched to a low impedance line. The frequency control of oscillator 2 was connected to the rotary condenser by a reduction gear, so that a pointer attached to the control spindle magnified the motion of the frequency dial by a factor of five. The pointer moved against one of a set of frequency scales, each calibrated for use with a particular frequency of the standard tone. The marks on the scale were numbered from 1 to 10 at equal frequency intervals, and the standard frequency was represented by a mark at the middle of the frequency range. The frequency intervals between the graduation marks were 100, 200 and 500 c/s, on different scales, the size of interval used in a particular test depending on the subject's ability to discriminate pitch.One moving-coil earphone was used, S.T. and C. type 4026A, and a dummy earphone excluded noise from the opposite ear. The earphones were supported by a spring headband, and were provided with sponge-rubber rings to ensure that they fitted closely on the ears.The attenuators were made by the author, to match the low impedance of the headphone. Each was variable in steps of 5 db, and gave up to 130 db of attenuation.

No abstract

1. The relative spectral sensitivity of larvae of Calliphora vomitoria and C. erythrocephala has been determined, using the maggot's natural tendency to travel towards the weaker of two opposing lights. 2. The response to light at each wavelength between 402 and 602 mµ was the same for the two species, within ±0.1 log unit, and the results are well fitted by the bell-shaped curve characteristic of a (hypothetical) visual purple, maximal at 504 mµ. 3. Reduction of the intensity of the white standard light by one log unit did not change the shape of the curve, and the extended wavelength range, between 362 and 640 mµ, shows no sign of a subsidiary maximum of sensitivity. The levels of illumination were 10-2 and 10-3 f.c., and the absolute threshold was found to be about 10-6 f.c. 4. The spectral sensitivity found here is compared with the results of other workers for muscid flies and larvae. It is concluded that one of the independent maxima of sensitivity found in the flies is a continuance from the larval state.