696 Comparative Morphology of Chordates 



cavity and the perilymph space. The periphery of this stapedial plate 

 is attached to the wall of the fenestra by a ring of tough fibrous tissue 

 which effectually closes the aperture but is sufficiently elastic to permit 

 of slight in-and-out movement of the stapedial plate. Near the fenestra 

 vestibuli is another aperture in the bone, the fenestra cochleae (or 

 rotunda). This fenestra is completely closed by a tough and elastic 

 membrane. It is to be noted that the fenestra cochleae abuts upon 

 the scala tympani of the bony cochlea, while the fenestra vestibuli 

 is at the base of the upper cochlear passage, the scala vestibuli. The 

 relations of these several spaces may be appreciated if it be imagined 

 that a person small enough to do so should make an exploratory tour 

 of the cochlea (Fig. 518). Starting at the "oval window," he could 

 ascend the scala vestibuli (scala meaning a "staircase," but the lamina 

 spiralis is, in fact, a spiral ramp rather than a flight of steps) and, 

 arriving at the apex of the cochlea, could climb around the free edge 

 of the lamina spiralis and descend the cochlea via the scala tympani, 

 finishing his trip at the "round window" (fenestra cochleae). This 

 relation of the passages is of physiologic significance. 



Certain very small muscles extend between the deeper part of the 

 wall of the tympanic cavity and the ear-bones. The tensor tympani 

 inserts on the malleus; the stapedius inserts on that end of the stapes 

 which articulates with the incus. Contraction of the tensor tympani 

 pulls the tympanic membrane inward and thus increases the tension 

 in it. Contraction of the stapedius tends to draw the stapes away from 

 the fenestra vestibuli and so increases tension in the membrane which 

 joins the stapedial plate to the rim of the fenestra. 



An externally conspicuous and distinctively mammalian addition 

 to the auditory apparatus is the external ear (pinna or auricula), 

 whose more or less funnel-like form enables it to collect and reflect 

 aerial vibrations into the external auditory meatus at its base. It is 

 supported by cartilage and equipped with muscles which, in most 

 mammals, make possible its rotation into positions favorable for 

 reception of vibrations. In relative size, form, and degree of mobility, 

 the pinna is highly variable, ranging from the disproportionately 

 large pinna of the elephant to complete absence of pinnae in mono- 

 tremes, sea cows, cetaceans, some seals, and some burrowing animals 

 (e.g., the Australian marsupial blind mole, Notoryctes). Projecting 

 pinnae are obviously incompatible with swimming and burrowing. 



In relation to the tympanic cavity and external ear, the tympanic 

 bone is a characteristic mammalian feature. This dermal bone sur- 

 rounds, partly or completely, the tympanic membrane and constitutes 

 the greater part of the bony wall of the tympanic cavity, the wall being 



