some amphibia; the articulation of the globe on a stalk of cartilage in the skate and shark; the curtain in the eye of the skate, which can be let down so as to cover the pupil; and the unique structure of the lobitis anableps, where the cornea is divided into two portions, and there is a double pupil with a single lens.

Insects.

Two kinds of eyes, very dissimilar in their structure, are found in this class. One sort is small and simple; the others, which are large, seem to consist of an aggregation of smaller eyes; for their general convexity is divided into an immense number of small hexagonal convex surfaces, which may be considered as so many distinct corneæ. The first kind is formed in different numbers in most of the aptera, as also in the larvæ of many winged insects. When these undergo the last or complete metamorphosis, and receive their wings, they gain at the same time the large compound eyes. Several genera of winged insects and aptera have stemmata besides their compound eyes.

The internal structure has hitherto been investigated only in the large polyedrous eyes. The back of the cornea is covered with a dark

pigment. Behind this are numerous white bodies of a hexagonal prismatic shape, and equal in number to that of the facets of the cornea. A second coloured membrane covers these, and appears to receive the expansion of the optic nerve. Further investigation is, however, required, in order to shew how these eyes enable the insect to see, and to determine the distinctions between two such very different organs.

Vermes.

The cuttle-fish only, of this whole class, has been hitherto shewn to possess true eyes, the nature of which cannot be disputed. They resemble, on the whole, those of red-blooded animals, particularly fishes; they are at least incomparably more like them than the eyes of any known insects, yet they are distinguished by several extraordinary peculiarities. The front of the eyeball is covered with loose membranes instead of a cornea; the iris is composed of a firm substance, which seems like a continuation of the sclerotic; and a process projects from the upper margin of the pupil, which gives that membrane a semilunar form. The corpus ciliare is very completely formed. In all

other vermes the eyes are entirely wanting, or their existence is very doubtful. Whether the black points at the extremities of what are called the horns of the common snail, are organs which really possess the power of vision, is still problematical.

CHAPTER II.

OF THE DISEASES OF THE EYE.

HAVING in the Introduction pointed out the circumstances which led me to direct my attention to diseases of the eye, and having, as in my work on the ear, described the parts of the eye essential to the perfection of its leading functions, I shall now proceed to examine the principal diseases of the eye: these, like those of the ear, may be classed according to the different parts of the organ in which they are seated, namely, external, comprehending inflammation of the eyelids, of the sclerotic coat and cornea, with the sequela of epiphora, ulceration, specks, and opacities; and internal, viz. cataract, cancer, and amaurosis or gutta

serena.

In so doing, I shall follow the anatomical arrangement, and consider the various morbid

F

affections of the different parts of this important organ and its appendages; commencing with the exterior, and carrying on my observations to the interior portions. I consider it better to follow the order of nature than to adopt any arbitrary arrangement, as there is not so clearly defined a line of demarcation between the external and internal parts of this organ as there is in the ear.

Diseases of the External Eye.

These, like the diseases of the external ear, depend on the nature of the structures affected. The reader may be reminded that the eyelids are partly cartilaginous and partly muscular, the tarsus being besides furnished with mucusglands, and the lachrymal gland furnishing a serous fluid.

I. Of Ophthalmia.

It may be correctly said, that inflammation affects equally all the membranes of the eye, exhibiting, in addition to the usual characteristics of general inflammation in other textures, great sensibility to light, and an increased flow