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 



Si 



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 ; Eu, Eustachian tube ; If, malleus ; J, incus ; St, stapes closing the 

 f enestra ovalis (Fo) ; Fr, fenestra rotunda ; F, 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 choroid 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 



