COMPARATIVE ANATOxMY. 
inisGS of the head, the olfactoi-y nerve, &c. 
is entirely wanting in these animals. 
ORGAN OF HEARING. 
Some Mammalia have not an external 
ear ; particularly such as live in tlie water, 
or under ground. 
Most quadrupeds have a peculiar hemi- 
spherical bony cavity, communicating with 
the tympanum, and seeming to hold the 
place of mastoid cells. 
The omithorhynchus, whose structure is 
in every respect so anomalous, has only two 
ossicula auditus. 
The cochlea, which belongs exclusively 
to tlie Mammalia, has in some cases one 
turn more than in man. 
Whales have an organ of hearing, but tlie 
parts are very small. 
Birds have no external ear ; only a single 
ossiculum auditus ; and a short, obtuse, hol- 
low, bony process, instead of cochlea. 
Reptiles have membranous semicircular 
canals and vestibulum ; generally a single 
ossiculum auditus resembling that of birds; 
and in some instances, a tympanum and 
inembrana tympani level with the surlace 
of the body. 
Fishes have a membranous vestibulum 
and canals ; but no external organs. 
THE ETE. 
A sensibility to the impressions of light 
is common to all those animals, which in a 
natural state are exposed to this element : 
it appears at least very evidently to exist in 
some of the most simple zoophytes, as the 
armed polypes (hydra) : but the power of 
perceiving the images of external objects is 
confined to those who are provided with 
eyes for their reception. Nature has be- 
stowed on some species even of red-blood- 
ed animals, a kind of rudiment of eyes, 
which have not the power of perceiving 
light: as if in compliance with some gene- 
lal model for the bodily structure of snch 
animals. This circumstance at least has 
been asserted of the blind rat (mannola ty- 
phlus) among mammalia ; and of the myx- 
ine glutinosa among fishes. 
The conjunctiva covering the front of 
the eye-ball, in the former animal is covered 
with hair, so that the eye, which is exceed- 
ingly small, seems to be completely useless. 
Large animals have small eye-balls in pro- 
portion to their size: this is very remark- 
ably the case with the whales. Those 
which are much under ground have the 
globe also very small; as the mole and 
tOL. II. 
shrew : in the former of these instances its 
existence has been altogether denied ; and 
it is not in fact larger than a pin’s head. 
The eyes of man and the simise are di* 
reeled forw^ards : in the latter animals in- 
deed they are placed nearer to each other 
than in the human subject. The lemur tai’- 
siiis has them more closely approximated 
than any other animal. All other Mamma- 
lia have these organs separated by a consi- 
derable interval, and directed laterally. 
The same circumstance obtains in birds, 
with the exception of the owl, who looks 
straiglit forwards. They are placed late- 
rally in all reptiles. Their situation varies 
much in fishes : they look upwards in the 
nranoscopus: they are both on the same 
side of the body in the pleuroneetes ; but 
in general their direction is lateral. 
The form of the globe varies according 
to the medium, in which the organ is to be 
exerted. In man and the mammalia, it de- 
viates very little from the spherical figure. 
In fishes it is flattened on its anterior part; 
in birds it is remarkably convex in front, 
the cornea being sometimes absolutely he- 
mispherical. The convexity of the crystal- 
line is in an inverse ratio to that of the cor- 
nea. Thus in fishes it is nearly spherical, 
and projects through the iris, so as to leave 
little or no room for aqueous humour : the 
cetacea, and those quadrupeds and birds 
which are much under water, have this part 
of the same form. The aqueous humour be- 
ing of the same density with the medium 
in which these animals are placed, would 
have no power of refracting rays of light, 
w'hich come through that medium : its place 
is supplied by an increased sphericity of 
the lens. In birds these circumstances are 
reversed : they inhabit generally a some- 
what elevated region of the atmosphere, 
and the rays which pass through this thin 
medium, are refracted by the aqueous hu- 
mour which exists in great abundance. 
Man, and the mammalia, which live on the 
surface of the earth, hold a middle place 
between these two extremes. 
The inner surface of the choroid coat, 
which in man is black tliroughout, is co- 
loured very beautifully on the temporal 
side of the eye in most quadrupeds, and this 
part is called the tapetum. 
The pigmentum nigrum is entirely defi- 
cient in the eye of the white rabbit, white 
ferret, &c. as well as in the variety of the 
human race called the albino. 
The quadrumana alone possess the fora- 
men centrale of the retina, besides man. 
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