. Comparative anatomy. Anatomy, Comparative. 598 COMPARATIVE ANATOMY 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 ti
. Comparative anatomy. Anatomy, Comparative. 598 COMPARATIVE ANATOMY 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. 501) The conclusion of morphologists that the vertebrate membranous ear is a modified lateral line organ, or a group of such organs, seems justified by the fact that the mem- branous ear develops, like a lateral line organ, from a thickened placode of ectoderm on the side of the head, and that its later ontogenetic changes resemble those of a lateral line organ. In both cases, the skin sinks below the surface, and patches of sensory cells are differentiated. Moreover, the eighth nerve develops as a branch of the facial, a nerve which supplies lateral line organs. In the elasmobranchs, the external apertures of the invagination canal of the statocyst lies near the openings of the occipital row of lateral line organs. A similar separation of lateral line organs also occurs in the case of the ampullae of Lorenzini and the vesicles of Savi in elasmobranchs. Both from their development and nerve relations these organs are obviously differen- tiated lateral line organs. The ear, it is be- lieved, has had a similar history. The Human Ear The ear consists of three parts, external, middle, and internal. The last is the true sensory organ, which has the double function of equilibra- tion and hearing. The External Ear. The external ear has two parts, an auricle or pinna, supported by cartilages, and an auditory meatus, which extends to the ear drum. Sound waves are
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