From the common retina the "secondary optic nerves" extend backwards in the same direction for a short distance to the bulbous enlargement of the "primary optic nerve", which itself is derived from the supra-æsophagal ganglion, corresponding to the brain in higher animals. Each of these hexagonal eye-tubes is thus constituted an independent ocellus, associated with the others only by a common dependence on the primary optic nerve. The superficial area of its larger extremity, or that presented to the surface of the eye, varies in the different orders of the Articulata. The number of ocelli likewise varies for the different orders, but is determinate in each order of a class; thus, in the common house-fly (Musca domestica) of the order Diptera, the number is 4,000; in the dragonfly (Libellula), order Neuroptera, about 12,000; in the butterfly (Papilio), order Lepidoptera, 17,355.

The peculiar construction of the organ just described, would appear to be supplemental of the immobility of the head and eyes, and also of the want of an apparatus for optical adjustment. The eye being immovably fixed in its position, and the head comparatively so, an extensive range of vision could be acquired only by conferring upon the eye such a degree of convexity and prominence as would enable it to command a view of objects placed before and around it; but a simple refractile organ of this form, and incapable of altering the direction of its axis, would obviously be attended with the inconvenience of spherical aberration in an exaggerated degree. To obviate this inconvenience, and render vision as distinct as the circumstances will admit, the eye is divided into a number of radiating cones, having separate and independent functions, and so directed that the rays of light reaching the eye from an object situated anywhere within the field of vision, must fall upon one or more of them in the direction of its proper axis;

and as each is encased in a dark choroid, and not more than the fraction of a line in its transverse diameter, the marginal portion of the entering pencil of rays, or all those which deviate from the axis, must impinge upon the sides of the tube, and be there absorbed; the consequence is, that the axial ray alone finds entrance, and as this suffers no refraction in its passage, spherical aberration cannot occur. A further advantage is gained by an eye of this construction in the power which it confers upon the animal of seeing the most minute objects with the utmost distinctness; for any object which subtends the produced axes of two adjacent ocelli, will be depicted upon the retina as distinctly as one of much larger size.

It is obvious, likewise, that rays of light reaching the eye from any distance within the field of vision, will find a sufficient number of ocelli suitably directed to receive and transmit them

in the line of their axes; and as none but axial rays can reach the retina, all others being arrested in their progress, distance can have no influence in modifying the definition of the object, save only as to the vividness of the impression, and hence aberration from parallax can have no existence, and an apparatus for adjustment is not required.

In man and vertebrate animals generally the eye is constructed upon one and the same model, and possesses essentially the same parts variously modified according to the medium in which the animal lives, and to some extent also the peculiar habits to which it is addicted. The mechanical principle held in view appears to be that of the camera obscura. The constituent parts of the organ may be conveniently divided into the essential and the tributary; the former include the refractile media, namely, the cornea, aqueous humour, crystalline lens, and vitreous body, together with the retina or percipient membrane; the latter, the sclerotic coat and choroid, with its appendages the ciliary ligament, ciliary body, ciliary muscle, and iris, besides the proper rotatory muscles and common elements of organization, viz., vessels and nerves.

The figure of the eye is, in itself, of but secondary importance, and entirely determined by that of the refractile media in the aggregate; and as the latter affect a spheroid for optical reasons sufficiently obvious, the eye assumes a corresponding form within certain limits. The limitation alluded to may be expressed by the formula-the greater the sphericity of the lens, and the shorter its consequent focal length, the less is the antero-posterior diameter of the eye-ball. But as the total magnitude of the vitreous body, which occupies four-fifths of the hollow sphere, must bear a certain fixed proportion to that of the lens for the purpose of optical adjustment, it follows that the eye must gain in the transverse what it loses in the antero-posterior axis. Hence, in fishes, whose lens is a perfect sphere, the eye is flattened considerably in the direction from before backwards, and in mammalia and birds it is more globular, whilst the lens is less spherical than in fishes. In man the figure of the eye-ball and crystalline lens is in strict accordance with the principle above laid down. The axis of the former is about .98 inch, and its vertical diameter .90 inch. The excess of the antero-posterior over the vertical diameter is due to the presence of the cornea in front, which, being a segment of a smaller sphere, having a radius of 3.3 lines appended to the larger, represented by the eye-ball,

1 Valentin's Physiology, by Brinton, p. 430.

Sömmering de oculorum hominis animaliuni que sectione horizontali commentatio. Göttingæ, 1818.

increases the antero-posterior diameter by about 4 inch, and renders the eye spheroidal. The axis of the lens measures 1.6 line, its transverse diameter, 3.6, radius of anterior convexity, 4.2, of posterior convexity, 2.4 lines. Thus the eye of man presents a configuration the reverse of that which characterises the organ in mammals, birds, and fishes. The convexity of the lens is to a certain extent supplemental of that of the cornea and aqueous humour taken together. In fish, the aqueous humour is small in quantity, because its refractive index being scarcely higher than that of the water in which the animal lives, such a fluid could serve no useful purpose as a refracting medium; it is therefore present in quantity barely sufficient to float and sustain the iris; and as the cornea has a degree of convexity corresponding to the quantity of the aqueous humour, it is in this class of animals nearly flat: hence the remarkable convexity of the crystalline lens. The eye of the bird affords an exception to this rule, namely, a convex cornea, an abundant aqueous humour, and a spheroidal lens. The rarity of the medium in which high-flying birds occasionally exercise the faculty of vision, as well as the distance at which they view objects whilst soaring in the higher regions of the atmosphere, require that every refractile agency of which the eye is capable should be placed at the animal's disposal for these occasions. But as the birds under consideration, in common with those of less exalted flight, ordinarily move in the lower and more dense strata of the atmosphere, the provision just mentioned must not be fixed and immutable, but rather capable of such modification at the will of the animal, as may adapt it to the special circumstances under which it is brought into operation. The ciliary muscle, which in birds is of the striated or voluntary kind, supplies the means of effecting the necessary change, by regulating the degree of convexity of the cornea, and altering the position of the crystalline lens.

A further provision for adapting the eye of the bird to vision at various distances, is supplied in the circle of bony plates imbedded in that portion of the sclerotic which immediately surrounds the cornea; for when, under the compressing action of the oblique muscles which embrace it like a girdle, the eye is elongated from behind forwards, the protrusion of its anterior surface is limited to the cornea by the unyielding nature of the osscous circle immediately adjoining, hence a greater convexity of the cornea, and a proportionately increased refraction of the rays of light transmitted through it.

3 Sömmering, opus citat.

Refractive index of aqueous humour, 1.337; do. of water, 1.336.

Fig. 1.

With the exceptions presented by man and the Quadrumana amongst mammals, and the strigida (owl-family) amongst birds, the eye occupies a position on the head more or less lateral in all animals. The eye-dots of Annelida and the "simple eyes" of insects and crustaceans will, of course, constitute additional exceptions; but the former being rudimentary and anomalous in so many particulars besides that of position, and the latter, when placed centrally on the head, being supernumerary organs, need not to be admitted as invalidating the rule just stated. In consequence of the lateral position of the eyes, the visual axes diverge in front when the eye is in equilibrio, and in the majority of animals do not admit of being brought to a focus, however much they may be produced anteriorly,

as is shown in fig. 1 (annexed), and fig. 2 (p. 481), which represent the relative position and direction of the eyes in oman and the ox. It is obvious that animals so endowed are incapable of perceiving a single object with both eyes simultaneously, in the lines of their visual axes, and as it will be shown subsequently that the convergence of the axes of vision upon an object, constitutes an indispensable condition for the single perception of it with two eyes, it follows that the great bulk of the animal creation, and probably the entire animal kingdom with the exceptions formerly mentioned, are destitute of this the highest attribute of the faculty of vision.

Fig. 2.

Fig. 3

B, entrance of optic nerve with central artery of retina.

In the bottom of
the human eye, at a
distance of about
one-eighth of an inch
outside and a little
above the point of
entrance of the optic
nerve, and nearly in
the axis of vision, a
yellow spot is observ-
able in the retina,
as represented at
A in the diagram
fig. 3. This spot,
which is about
inch in diameter, was
described by

first
Sömmering,

and

named by him the "foramen centrale", from the circumstance that a minute aperture is visible in it, and that it occupies a central position in the eye; over it the vesicular constituent of the retina alone is expanded, the fibrous and vascular elements diverging as if to avoid it." The yellow spot of Sömmering is peculiar to man and the monkey tribe, and forms a zoological feature quite distinctive of the orders Bimana and Quadrumana; it therefore coincides with the frontal po

4 Todd and Bowman, the Physiological Anatomy and Physiology of Man, part iii. p. 31.