110 EXPERIMENTAL PHYSIOLOGY 



The production of sounds by vibration of the vocal cords, and the dependence 

 of the pitch of sounds on the tension of the cords. Take a sheep's larynx and 

 tie a glass tube into the trachea. Fix the larynx securely on a board, with the 

 dorsal surface downwards, by wires or strong pins through the cricoid cartilage 

 and epiglottis. Pass a string through the lower part of the thyroid cartilage. 

 When this string is pulled vertically upwards, the vocal cords are stretched in 

 proportion to the pull, and become approximated. If air is blown through the 

 tube in the trachea, their edges are set in vibration when thus approximated 

 and a sound is emitted, the pitch of which varies with the tension of the cords 

 (the blast of air should be of about the same strength throughout). 



Analysis of sounds of musical instruments and of the voice. The analysis 

 can be made by applying different Helmholtz resonators to the ear of the observer. 

 Konig's manometric flames, which are provided with these resonators and are 

 examined with the aid of vertical rotating mirrors, are also employed for this 

 purpose. 



Determination of range of pitch for audition. The highest and lowest notes 

 which can be appreciated are determined by the use of Galton's adjustable 

 whistle. 



Conduction of sound by the bones of the skull. Stop the ears with wool. Set 

 a small tuning-fork in vibration, and hold it with the base touching the top of 

 the skull. The sound is propagated to the cochlea by the bones of the skull. 



Inspection of the tympanic membrane. Using a mirror with a central aperture 

 fixed in front of the eye, throw the reflection of a lamp into the meatus of the 

 subject, whose external ear must be drawn somewhat backwards and upwards. 



Propagation of sound to the internal ear. The mode of transmission may be 

 st udied with the aid of a model showing the bones of the middle ear and their 

 attachments to the membrana tympani and the fenestra ovalis. The model 

 shows that when the tympanic membrane to which the handle of the malleus 

 is attached is driven in, the base of the stapes, which fits into the fenestra 

 ovalis, follows the movement : but when the tympanic membrane is forced 

 outwards beyond a certain point the stapes is not dragged after it owing to the 

 nature of the articulation between malleus and incus. The model also shows the 

 effect of the tensor tympani in pulling inwards the handle of the malleus and with 

 it the membrana tympani, and the effect of the stapedius in pulling the head of 

 the stapes backwards and causing the base to be tilted within the fenestra 

 ovalis, thus rendering tight the ligament which fixes it in that aperture. 



Semicircular canals. For demonstrating the effects of injury to semicircular 

 canals, a bird (pigeon) is employed. An opening is made in the side of the 

 .skull of the anaesthetised animal with a very small trephine, and through the 

 aperture a special instrument is introduced and passed underneath the dura 

 mater until one of the bony semicircular canals (which in the bird project above 

 the surface of the petrous bone) is met with ; the canal can then be broken across. 



For observing the effects of stimulation of the canals a strong galvanic 

 current is passed from one side of the head to the other; pad electrodes wetted 

 with strong salt solution being applied in the neighbourhood of the ears : the 

 stimulation occurs on the side of the kathode. 



