34 
—only about the 1-120th of an inch in thickness,—has five lay 
and while the other parts of the eye fulfil a mechanical or phy: 
office, its function is vital or mental. 
It was necessary tc dwell longer on the compound eye of the 
invertebrates, as our knowledge of its structure is still imperfect. 
Little advance had been made for 200 years until the German 
physiologists lately showed that the eyes of insects and erusta- 
. ceans— with a common functidu—pres. __ _ many homolog as vith 
those of vertebrates. 
~ Ades 
In the eye of the dragon-fly, the bulbed édfids of the 
nerve fibres are not, as has been supposed, crystalline lenses, 
but true representatives of the rods and cones of the _ baccil- 
lary layer of the retina—the percipient element of the eye. 
These bulbed nerves are each covered with pigment, except 
that, where they come in contact with the lenses, there are aper- 
tures representing those of the vertebrate pupil. The lenses are 
the outer hexagonal facets, each a double convex lens of itself, the 
inner surface the most convex, and placed in contact with a bulbed 
nerve, each with its independent nerve forming a distinct eye. 
The facets in the compound eyes of insects can be numbered by 
hundreds, but as, with the human eyes, the two images are per- 
ceived as one, so the mental perception of the fly recognizes but a 
single object. 
There was an absence in the eyes of insects of the crys- 
talline lens and vitreous humor of the vertebrates, although 
the essential lens and cones are represented. the n. merous facets 
of the eyes of insects allow, from their positidn; of extreme range 
of vision, and render unnecessary the powerful muscles of the 
vertebrate eye. The eyes of insects were most fitly placed and 
formed. With reference to their organization generally, organs 
should always be considered with reference to general structure. 
In the single eyes of insects, placed between the com- 
pound eyes, and in those of spiders, he conceived that the spheri- 
cal portion at the back of the single eye was an enlargement of 
the cornea, and not a vitreous humor. 
He then exhibited, under the microscope, beautiful sections 
of the eyes of insects, in demonstration of his subject, which was 
well illustrated by diagrams, 
ll Lee 
