Mammalia: Nervous System 697 



completed by the deeper petrosal bone. The tympanic bone commonly 

 forms a conspicuous semiglobular swelling, the bulla tympani (Fig. 

 465, T), protruding externally at the base of the otic region of the skull. 

 It provides a firm support for the tympanic membrane. In some cases 

 it produces a thin-walled tubular shell enclosing the external auditory 

 meatus and giving support to the base of the pinna. In the rabbit, for 

 example, both this tubular part and the bulla are conspicuous. In 

 monotremes, marsupials, and some placental mammals, the tympanic 

 appears as a distinct bone in the adult skull, but in most mammals it 

 fuses into the temporal complex. 



The mammalian tympanic bone has been regarded as the homolog 

 of the reptilian quadrat ojugal, which does not otherwise appear in 

 mammals, but in the embryo the tympanic is in such relation to the 

 malleus (reptilian articular) as would rather identify the tympanic as 

 equivalent to the reptilian dermal angular. 



Hearing and Equilibration 



The sequence of events resulting in stimulation of the sensory cells 

 of the organ of Corti is as follows: Vibrations of the external air, more 

 or less concentrated into the meatus by the pinna, impinge upon the 

 tympanic membrane. The texture and state of tension of this mem- 

 brane are such that it has little or no tendency to vibrate at any fixed 

 rate determined within itself, but freely vibrates in unison with the 

 external medium. The tympanic cavity is filled with air and com- 

 municates with the external air via the Eustachian tube and nasal 

 passage. This arrangement equalizes atmospheric pressure on the 

 opposite sides of the tympanic membrane, thus allowing it perfect 

 freedom of vibration. As a result of the malady known as a "common 

 cold," the Eustachian tube may become obstructed, a condition which 

 more or less impairs hearing. 



The vibrations of the tympanic membrane are transmitted by the 

 chain of auditory ossicles to the wall between the tympanic cavity and 

 perilymph space of the internal ear, causing the stapedial plate to 

 vibrate in and out in unison with vibrations of the tympanic mem- 

 brane. This it is able to do because of its elastic attachment to the rim 

 of the fenestra vestibuli (ovalis). The part played by the two muscles, 

 tensor tympani and stapedius, is not certainly known. It is possible 

 that, by controlling the degree of tension in the tympanic membrane 

 and in the membranous attachment of the stapes, they may adjust 

 the transmitting mechanism so as to produce an optimum response to 

 the particular vibrations which are coining in, thus acting as a mecha- 

 nism for auditory " accommodation." These muscles are striated but, at 

 least in man, are usually involuntary. The rare individual in whom 



