Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 284 MAMMALIA. are usually placed at the sides of the head in an incompletely closed orbit (continuous with the temporal fossa). As a rule, each eye has a separate field of vision; a convergence of the optic axes is only possible when the eyes are placed on the front of the head (Primates}. Besides the upper and lower eyelids there is an internal nictitating membrane (with the Harderian gland), which is, however, not fully developed, and is without the muscular apparatus of the Birds' nic
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 284 MAMMALIA. are usually placed at the sides of the head in an incompletely closed orbit (continuous with the temporal fossa). As a rule, each eye has a separate field of vision; a convergence of the optic axes is only possible when the eyes are placed on the front of the head (Primates}. Besides the upper and lower eyelids there is an internal nictitating membrane (with the Harderian gland), which is, however, not fully developed, and is without the muscular apparatus of the Birds' nictitating membrane; it is sometimes reduced to a small rudiment (plica semilunaris) at the inner corner of the eye. The eyeball is more or less spherical (in the Cetacea, etc., with shortened axis), and St FIG. 672.—The human ear (combined representation) with view of the tympanic membrane from the tympanic cavity. Ge, External auditory meatus ; T, membrana tympani ; Ct tympanic cavity; J?«, Eustachian tube; 31, malleus; J, incus; St, stapes closing the fenestra ovalis (Fo) ; Fr, fenestra rotunda; I', vestibule; C, cochlea; S, semicircular canals. can often be retracted into the orbit by a retractor bulbi. The lachrymal gland with its duct, which opens into the nasal cavity, lies on the upper and outer side of the orbit. The choroicl has a tapetum in the Carnivores, Pinnipedes, Dolphins, Ungulates, and some Marsupials. The auditory organ (fig. 672, and fig. 578, iii.) differs from that of the bird principally in the more complicated development of the external ear, in the greater number of sound-conducting bones (stapes, incus, malleus), and in the form of the cochlea, which is usually coiled into two or three spiral passages. The tympanic cavity is also more spacious, and is by no means always confined to the space
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