COMPARATIVE ANATOMY. 
Most mammalia possess a membrana nic- 
titans, or third eyelid, behind which the 
eyeball can be drawn, when offended by any 
extraneous matter. 
Birds are distinguished by having a bony 
ring, composed of numerous flat and over- 
lapping thin plates^ in the substance of the 
sclerotica, at its anterior part. 
Another great peculiarity consists in the 
marsnpium or pecten, which appears as a 
large folded process of the choroid, coming 
through the retina at the back of the eye, 
and running in the substance of the vitreous 
humour towai’ds the crystalline lens, which 
it does not quite reach. 
The third eyelid, or membrana nictitans 
of birds, is a thin semi-transparent fold of 
the conjunctiva ; which, in the state of rest, 
lies in the inner corner of the eye, with its 
loose edge nearly vertical, but can be drawn 
out so as to cover the whole front of the 
globe. By this, according to Cuvier, the 
eagle is enabled to look at the sun. 
It is capable of being expanded over the 
globe of the eye by the combined action of 
two very singular muscles, which are at- 
tached towards the back of the sclerotica. 
One of these, which is called from its shape 
the quadratus, arises from the upper and 
back part of the sclerotica; its fibres 
descend in a parallel course towards 
the optic nerve, and terminate in a semi- 
circular margin, formed by a tendon of a 
very singular construction ; for it has no in- 
sertion, but constitutes a cylindrical canal. 
The second muscle, which is called the py- 
ramidalis, arises from the lower and back 
part of the sclerotica towards the nose. It 
gives rise to a long tendinous chord, which 
runs through the canal of tlie quadratus, as 
in a pulley. Having thus arrived at the ex- 
terior part of the eyeball, it runs in a eellu- 
lar sheath of the sclerotica along the under 
part of the eye, to the lower portion of the 
loose edge of the membrana nictitans, in 
which it is inserted. 
■ By the united action of these two mus- 
cles, the third eyelid will be drawn towards 
the outer angle of the eye, so as to cover 
the front of the globe ; and its own elasti- 
city will restore it to its former situation. 
• Two kinds of eyes, very dissimilar in 
their structure, are found in insects : one 
sort is small and simple, (stemmata) ; the 
others, wliich are large, seem to consist of 
'an aggi'egation of smaller eyes ; for their ge- 
neral convexity is divided into an immense 
number of small hexagonal convex sur- 
faces, which may be considered as so many 
distinct come®. Tlie first kind is found in 
different numbers in most of the aptera, as 
also in the larva of many winged insects. 
When these undergo the last or complete 
metamorphosis, and receive their wings, 
they gain at the same time tlie large com- 
pound eyes. Several genera of winged in- 
sects and aptera (as the larger species of 
monoculi), have stemmata besides their 
compound eyes. 
The internal structure has hitherto been 
investigated only in the large polyedrous 
eyes.. The hack of the cornea (which is 
the part divided in front into the hexago- 
nal surfaces, called in French, facettes) is 
covered with a dark pigment ; behind this 
are numerous white bodies, of an hexago- 
nal prismatic shape, and equal in number 
to that of the facettes of the cornea. A se- 
cond coloured membrane covers these, and 
appears to receive the expansion of the op- 
tic nerve. 
Further investigation is, however, re- 
quired, in order to shew how these eyes en- 
able the insect to see ; and to determine 
the distinction between two such very dift 
ferent organs. 
MUSCLES. 
The nature and objects of the present 
work render it impossible for us to enter 
into the details of comparative myology ; 
we shall therefore restrain our remarks to 
one or two subjects. 
The differences which we discern in the 
muscles of the lower extremity between 
man and the other mammalia, arise out of 
that characteristic feature which so strik- 
ingly distinguishes man from all other ani- 
mals, viz. his erect stature. The most mi- 
nute investigation of this subject will shew 
us that the erect position belongs to man 
only ; and thereby confirms the elegant ob- 
servation of the Roman poet : 
Promque cum spectent animalia cetera ter- 
ram, 
Os homini sublime dedit ; ccelumque tueri 
Jussit; et erectns ad sidera tollere vultus. 
In order to enable any animal to pre- 
serve the erect position, the following 
conditions are required. 1st. That the 
parts of tlie body should be so disposed as 
to admit of being maintained with ease 
in a state of equilibrium. 2dly. That the 
muscles should have suflicient power to cor- 
rect the deviations frpm tliis state. 3dly. 
That the centre of gravity of the whole 
body should fell within the space occupied 
