. Chordate anatomy. Chordata; Anatomy, Comparative. 4i8 CHORDATE ANATOMY In reptiles the lagena and basilar papilla unite in an elongated cochlea, which in crocodiles becomes spirally wound. By the attachment of the cochlear duct to the bony labyrinth along two sides, the perilymphatic cavity is divided into two portions, the scala vestibuli and the scala tympani. Further advance towards the mammalian ear is seen in the appearance of a membrane-covered window, the fenestra vestibuli, to which the stapes is attached. As a result of these advances, the hearing of reptiles is noticeably keener th
. Chordate anatomy. Chordata; Anatomy, Comparative. 4i8 CHORDATE ANATOMY In reptiles the lagena and basilar papilla unite in an elongated cochlea, which in crocodiles becomes spirally wound. By the attachment of the cochlear duct to the bony labyrinth along two sides, the perilymphatic cavity is divided into two portions, the scala vestibuli and the scala tympani. Further advance towards the mammalian ear is seen in the appearance of a membrane-covered window, the fenestra vestibuli, to which the stapes is attached. As a result of these advances, the hearing of reptiles is noticeably keener than that of creatures lower in the scale. In mammals, the length of the cochlea varies from a half-turn in Echidna to three and a half turns in the deer. The keenness and range. Fig. 367.—The left membranous labyrinth of vertebrates as seen in lateral view. A, Myxine; B, Petromyzon; C, teleost; D, frog; E, crocodile; F, bird; G, mammal. The series represents fairly closely an evolutionary series. (Redrawn after Hesse.) of the sense of hearing differ correspondingly in the two animals. In correlation with the elongation of the cochlea, the length of Corti 's organ is increased and, with it, the range of audible sounds. A second mem- brane-covered window, the fenestra cochleae, is added to the inner ear. The malleus and incus are added to the stapes, to form a chain of bones so arranged that the amplitude of the vibrations of the ear drum is reduced and their intensity increased as they pass from the drum to the fenestra vestibuli. The efficiency of the apparatus is increased by the addition of two muscles, the stapedial, the smallest of skeletal muscles, and the tensor tympani which tightens the drum. (Fig. 164) The conclusion of morphologists that the vertebrate membranous ear is a modiiied lateral-line organ, or a group of such organs, seems justified by the fact that the membranous ear develops, like a lateral-line organ,. Please note that these images are extracted from sca
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