. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 794 Biology of the Vertebrates In amphibians, instead of a chain of three bones, there is a single rodlike columella, extending from the ear drum directly to the fenestra vestibuli (Fig. 694a) . This device forwards vibrations from the drum to the inner ear with equal power and amplitude, instead of with an increased thrust and lessened amplitude as is the case with the chain of ear bones in mammals. There may be two parts to the columella in reptiles and
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 794 Biology of the Vertebrates In amphibians, instead of a chain of three bones, there is a single rodlike columella, extending from the ear drum directly to the fenestra vestibuli (Fig. 694a) . This device forwards vibrations from the drum to the inner ear with equal power and amplitude, instead of with an increased thrust and lessened amplitude as is the case with the chain of ear bones in mammals. There may be two parts to the columella in reptiles and birds, either jointed or fused with each other (Fig. 694b ). The inner element, which is homologous with the stapes of mammals, is the plectrum, while the outer part close to the drum is called the extra-columella. Pterygoquadrate Stapes (Columella) \ Squamosal / ^Foot-Plate of \ \ _--—'/' T^""' Columella y^\^ K(Z£.''rz-£''--—'Operculum f "£^^^T9-Scapula *-,-«-.«- \^ P/^Quadrate JVST ^V ~ScaPula Opisthotic n^^v ^=2^2£/ YC~> V J^\ ''m ^Tympanic Cavity ,^/~ \ „ )$)L-Opercular &^^lW -ill^Plectrum Mandible JT^j I Muscle J^v/M JM r • u u >S^J4/^^^*: -r-Tympanic Membrane Sacculus—^V/^^^^#iH _\ _ . 'I Eustach B Fig. 694. Ear bones of urodeles (a) and turtles (b). (a, after Kings- bury and Reed; b, after Plate.) In monotremes and marsupials the stapes remains cylindrical and solid at the enlarged inner end, like a pestle, but in other mammals it is pierced by a hole through which a blood vessel passes, so that it takes on a fancied resemblance to a stirrup, hence its name. Two tiny muscles lie within the tympanic cavity. The tensor tympani, attached to the malleus and supplied from the Vllth cranial nerve, regulates the tension of the tympanic membrane. The stapedius, which is fastened to the stapes and supplied from the Vth cranial nerve, adjusts the fitting of the stapes in the fenestra ovalis. The ear bones have a dramatic origin, being made
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
