. Chordate anatomy. Chordata; Anatomy, Comparative. THE SENSE ORGANS 419 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 aperture of the invagination canal of the statocyst lies near the openings of the occipital row of lateral-line organs. A similar separa- tion of
. Chordate anatomy. Chordata; Anatomy, Comparative. THE SENSE ORGANS 419 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 aperture of the invagination canal of the statocyst lies near the openings of the occipital row of lateral-line organs. A similar separa- tion of lateral-line organs occurs also 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 differentiated lateral-line organs. The ear, it is believed, 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 equilibration 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 collected by the pinna and conveyed by the meatus to the ear-drum which lies about an inch below the surface of the head. The , , . vestibular nerve and its gan- deeper portion of the auricle, which forms a giion. (From Kingsley's vestibule to the meatus, is the concha. "Comparative Anatomy of . Vertebrates," after Streeter.) The opening of the meatus is guarded by two processes, a ventral tragus next the cheek and a dorsal antitragus opposite. The incurved outer rim of the auricle is the helix. The antihelix is a smaller ridge which bounds the concha dorsally. The walls of the meatus are supported laterally by fibro-cartilage and medially by bone and lined by skin„ The meatus is beset with hairs, and contains many tubular glands which secrete the ear-wax.
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Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
