affords a non-reciprocating cavity for the free vibration of its membrane and of the otosteals: it also renders the labyrinth independent of atmospheric vicissitudes. The otosteals conduct vibrations from the tympanic membrane to the vestibular one, and, under the influence of the muscles, regulate the tension of both these and of the cochlear fenestra, so as to protect the ear against the effects of sounds of great intensity. The external ear and meatus are collectors and conductors of vibrations, and the former assists in enabling us to judge of the direction of sounds.

§ 217. Organ of Sight.-A. Eyeball. The organ of sight, like that of smell, is wanting in a few Mammals, the eyeball being reduced to the size and condition of the 'ocellus' in Amblyopsis, and to its simple primitive office of taking cognisance of light, a filament of the fifth aiding the remnant of a proper optic nerve. The moles, especially the Italian kind, Talpa cæca, and mole-rats, exemplify this condition, in which, as in Spalar typhlus, the skin passes over the ocellus without any palpebral opening, or loss of hair. The eyeballs are very small in the allied genus Bathyergus, fig. 174, and other rodent burrowers: they acquire the largest absolute and proportional size in the Ruminant order. In no Mammal is bone developed in the

193

a

Diagrammatic section of Mammalian eye. cv".

sclerotic in most a special cavity, called 'orbit,' is fashioned in the facial part of the skull to give lodgment to the eye-ball. One sees least indication of it in the blind quadrupeds above noted and in the ant-eaters: it is deepest, best defined, and most completely walled in Man.

In all Mammals with the eye developed for sight, properly so called, we recognise, as in the diagrammatic section, fig. 193, the fibrous capsule, a, called 'sclerotic coat,' the transparent fore part, b, called cornea;' the vascular tunic, c, called choroid coat, becoming thickened, at d, by the so-called ciliary ligament,' from which the ciliary processes' are, as it were, reflected backward upon the capsule of the lens, f: while the movable curtain, oriris,' is continued onward into the space between b and f, leaving a central opening, called 'pupil,' for the admission of

light. The choroid, c, is lined by the expansion of the optic nerve called retina,' which extends to the ciliary processes,' and is kept outstretched by the vitreous humour' contained in the cells of the delicate membrane called hyaloid,' which restrains its forward advance beyond the crystalline humour' or lens, f. The space in front of this body is occupied by the aqueous humour,' and is divided by the iris into an anterior' and 'posterior chamber.'

The rays of light admitted by the cornea and pupil are slightly refracted in traversing the aqueous humour, and are subject to a greater degree of convergence in passing through the

194

Diagram of course of luminous rays in traversing the humours of the eye.

denser lens, fig. 194; when, striking the retina at the back of the globe, they there depict the image of the visual object, inverted.

In crepuscular and nocturnal Mammals (Pteromys, Aye-aye, Lemur) the cornea gains in size and convexity and the iris in breadth; the latter being capable of admitting many rays through a very wide pupil, which also it can completely close against the glare of noontide. The convexity of the lens is concomitantly increased, and it approaches the spherical form most nearly, in bats and nocturnal rodents. The vitreous humour is less in proportion to the crystalline and aqueous humours in such eyes. In aquatic Mammals, on the contrary, the cornea hardly projects (seals, whales), and there is little aqueous humour; here, also, the convexity of the lens is in excess, fig. 195, d. In most diurnal and terrestrial mammals, the eyeball is subspherical, the cornea slightly projecting at the fore part, as forming part of a smaller sphere than the rest of the globe. The lens retains much of the proportions shown in fig. 194.

In the Ornithorhynchus the eyeball is small and spherical; the sclerotic fibro-cartilaginous, the cornea flabby, the retina thick: there is no trace of pecten or marsupium: the lens is two lines in transverse diameter, one line in antero-posterior diameter; the anterior surface is nearly flat, the posterior very convex. The choroid is black, without a tapetum lucidum; the pupil is circular.

The anatomy of the eye offers no peculiarity illustrative of the affinities of the Marsupialia or of any other speciality in their economy save the nocturnal habits of the majority of the order. It is in relation to these habits that the lens is large and convex, the iris broad, the pupil round and very dilatable, and the cornea correspondingly large. The eye is relatively large in the swiftmoving, far-ranging Kangaroos: I found the dark pigment on both the inside and outside of the choroid; the ciliary processes are long: the lens is proportionally large. In the dead Kangaroo the radiated muscle of the iris is much contracted, and the pupil widely open. The eye is small in Didelphis virginiana; the pupil is round: the lens very convex.

The Insectivora have small eyes: the moles least of all. In a great pipe-toothed shrew (Solenodon) one foot in length, exclusive of tail, the palpebral opening does not exceed three lines, and there is no distinction between orbit and temporal fossa. Bats have the smallest eyes of all volant Vertebrates. In Rodents the size of the eyeball bears relation to the extent and swiftness of locomotion, and is greatest in Jerboide and Leporida. The position of the eyes is always lateral, and by the prominence of the cornea they are susceptible in these timid quadrupeds of receiving the image of a pursuer. In the hare and other rodents the retina seems to expand from the divisions of a cleft termination of the optic nerve, within the eyeball. The pupil is round in most Rodents: in a dead Agouti it was a horizontal ellipse. In the squirrel the ante-retral diameter of the eyeball is to the transverse as 11 to 12: in the hare it is as 23 to 25.1 In all the order Bruta the eyes are relatively small: in the sloths the contracted pupil is a vertical slit.

In Cetacea the eyes are small, especially in relation to the bulk of the larger kinds: and the essential part of the organ is still less, owing to the thickness of the sclerotic, fig. 195, a, a, and this increases from the cornea, b, backward to the long,

A table of these dimensions of the eye in different Vertebrates will be found in XI. iii. p. 390; also in cvi".

195

infundibular canal for the optic nerve, f. Outwardly the eyeball is subspherical; but, in the section figured, the contour of the cavity containing the vitreous humour, e, and lens, d, presents an ellipse, with the long axis transverse: in a Balanoptera of 65 feet in length, this axis measured 23 inches, and the shorter axis 2 inches: the posterior curve is regular; but, toward the cornea, the sclerotic turns in quickly, c, flattening the fore part of the eye: the distance between the fore part of the sclerotic and the bottom of the eye being but 14 inches. In shape the cornea is a longer ellipse than the eyeball, and the upper border is more curved than the lower: it is thinner at the centre than the circumference, and is soft and flaccid in the dead whale. The choroid has a silvery or bluish white hue on the inner surface: the darker pigment is limited to the ciliary processes and back of the iris. In a mysticete whale (Balana) the cellulosity connecting the choroid with the sclerotic was of a light brown hue: the darker pigment extends from the ciliary processes a little way upon the choroid: and in both kinds of whale is so disposed as to absorb the rays of light and prevent them being a second time reflected so as to disturb the spectrum on the back of the retina. Of the numerous minute folds which constitute the ciliary zone every third, fourth, or fifth is enlarged, and produced forward to form a wrinkled corrugated process about three lines long, compressed and terminating obtusely the intermediate shorter processes are of varying length; the long ciliary processes are about seventy in number, in Balanoptera. The peripheral radiated contractile fibres of the iris, and the central circular ones, are conspicuous on the back part of that curtain in whales: the front surface shows the wavy vessels radiating from arterial canals which surround the margin of the pupil which is transversely elliptical. Four equidistant canals in the thick sclerotic give passage to the long ciliary arteries and the vorticose veins: the two arteries which advance in the direction of the long axis of the pupil terminate in a canal bordering the pupil a little way from its margin : the wavy branches radiate from this canal, and are prominent on the

anterior surface of the iris. The quantity of the aqueous humour is small: the lens, d, is subspherical, flatter in front than behind. The nucleus is seen in the posterior half and the surrounding laminæ are reflected inward and backward toward the middle of the anterior surface of the nucleus, leaving a funnel-shaped cavity in front of it which is filled by less dense substance. In Hyperoodon the pupil is transversely oblong with a moderate projection of the upper margin, reminding one of the skate's pupillary curtain (vol. i. p. 334). In the Grampus the choroid presents a greenish tinge: in the Porpoise it is a bluish white. In both, the pupil resembles that of Hyperoodon. The retina is thick.

In the Seals the sclerotic is chiefly remarkable for the sudden thinning at the part corresponding with the ciliary zone; it is moderately thick both in front and behind: the cornea is thin and flabby. The muscles of the eye-ball being inserted into the anterior part of the sclerotic may shorten the axis of the eye and bring the lens nearer to the back of the globe, thus adapting it to vision in air and water. In the Sirenia the eye is very small. In a Rhytina of 25 feet in length the eye-ball was but 14 inch in diameter: it is about 1 inch in diameter in the Dugong: the pupil is circular.

The eye of the Elephant is about 2 inches in diameter, reminding one of that in the Whale by its small relative size: there is likewise an unusual thickness of fibrous or sclerotic substance at the entry of the optic nerve, and a similar extent of light-coloured tapetum within the choroid, which tapetum presents the fibrous type of structure: the pupil is round, the cornea is larger and more convex than in Cetacea.

In the Rhinoceros the eyeballs are of small comparative size; in the Indian species which I dissected,' each measured in antero-posterior diameter one inch five lines, and in transverse diameter one inch three lines. Some dark-brown pigment lies under the conjunctiva for the extent of about a line from the circumference of the cornea: the same kind of pigment is also deposited upon the outside of the nictitating eyelid, and over a great part of the inner surface of the same part, covered of course by a reflection of the conjunctiva. The trunks of the venæ vorticosa perforate the sclerotica half-way between the entry of the optic nerve and the edge of the cornea: their disposition, with the flocculent but somewhat firm connecting tissue of their radiating branches, presented that structure which most nearly resembled the figures given by Mr. Thomas of the parts he

1 v", p. 56.