ORGANS OF SENSE 73 
expansion of the optic nerve lining the posterior part of the ball. 
A portion of the fibro-vascular and highly pigmented layer, the 
choroid, which is interposed between the retina and the outer 
sclerotic coat, is in many mammals modified into a brilliantly- 
coloured light-reflecting surface, the tapetum lucidum. There is 
never a pecten or marsupium like that of the Sauropsida, nor is 
the sclerotic ever supported by a ring of flattened ossicles, as is so 
frequently the case in the lower vertebrated classes. The eyeball 
is moved in various directions by a series of muscles—the four 
straight, two oblique, and, except in the higher Primates, a pos- 
terior retractor muscle called choanoid. The superior oblique muscle 
passes through a tendinous pulley fastened to the roof of the orbit, 
which is a feature not found beyond the limits of the mammalian 
class. The eye is protected by the lids, generally distinctly separated 
into an upper and a lower movable flap, which, when closed, meet 
over the front of the eye in a more or less nearly horizontal line ; 
but sometimes, as in the Sirenia, the lids are not distinct, and the 
aperture is circular, closing to a point. In almost all mammals 
below the Primates, except the Cetacea, a “nictitating membrane” 
or third eyelid is placed at the inner corner of the eyeball, and 
works horizontally across the front of the ball within the true lids. 
Its action is instantaneous, being apparently for the purpose of 
cleaning the front of the transparent cornea ;—a function unneces- 
sary in animals whose eyes are habitually bathed in water, and which 
in Man and his nearest allies is performed by winking the true 
eyelids. Except in Cetacea the surface of the eye is kept moist by 
the secretion of the lachrymal gland, placed under the upper lid at 
its outer side, and the lids are lubricated by the Harderian and 
Meibomian glands, the former being situated at the inner side of 
the orbit, and especially related to the nictitating membrane, the 
latter in the lining membrane of the lids. 
Hearing.—The organ of hearing is inclosed in a bony capsule 
(periotic) situated in the side of the head, intercalated between the 
posterior (occipital) and the penultimate (parietal) segment of the 
skull. It has, in common with other vertebrates, three semicircular 
canals and a vestibule, but the cochlea is more fully developed than 
in the Sauropsida, and, except in the Monotremes, spirally con- 
voluted. The tympanic cavity is often dilated below, forming a 
smooth rounded prominence on the base of the skull, the auditory 
bulla (Fig. 8). The three principal ossicles, the “malleus,” “ incus,” 
and “stapes,” are always present, but variable in characters. In 
the Sirenia, Cetacea, and Seals they are massive in form, being in 
the first-named order of larger size than in any other mammals. In 
the Cetacea the malleus is ankylosed to the tympanic ; but in other 
mammals it is connected only with the membrana tympani. The 
stapes in the lower orders—LEdentates, Marsupials, and Monotremes 
