Fig. 408. Brain of the Rabbit.

* Curier, Lemons d'Anat. Comp. ii. p. 673.

Nevertheless the tubercula quadrigemina (fig. 408, d d) occupy the same relative position as in the Tortoise (vide fig. 348, b, c, e), and in like manner still give origin to the nerves appropriated to the instruments of sight, of which they are the proper ganglia.

(2373). The two optic nerves, before passing to their final destination, partially decussate each other, as in the human subject; they then proceed forward into the orbit, and, penetrating the globe of the eye, expand into the retinae.

(2374). Minutely to describe the construction of the eyeball in the Mammalia would be quite superfluous, seeing that in every essential particular it exactly corresponds with that of Man. The disposition of the sclerotic and choroid coats, the structure of the cornea, the arrangement of the humours and of the retina, the organization of the iris - in short, the whole economy of the eye is the same throughout the entire class. Nevertheless there are a few points of secondary importance deserving our attention, whereby the organ is adapted to peculiarities of circumstances in which different tribes are placed.

(2375). In the Cetacea, and also in the amphibious Carnivora that catch their prey in the water, the shape of the lens is nearly spherical as in Fishes; and the an tero-posterior diameter of the eye is in consequence considerably diminished by the extraordinary thickness of the sclerotic at the posterior aspect of the eyeball, - an arrangement approaching very nearly to that already described (§ 1809).

(2376). Instead of the dark-brown paint which lines the choroid of the human eye, in many Mammals the Ruyschian tunic secretes a pigmentum, of various brilliant hues, that shines with metallic splendour. This membrane, called the "tapetum," partially lines the bottom of the eyeball; but its use has not as yet been satisfactorily pointed out.

(2377). The shape of the pupil likewise varies in different quadrupeds: for the most part, indeed, the pupillary aperture is round, as it is in Man; but in Ruminants, and many other Herbivora, it is transversely oblong. In the Cats (Felidas), that hunt in the gloom, and consequently require every ray of light that can be made available, the pupil is a long vertical fissure: but this only obtains among the smaller genera; for in those Feline Carnivora that surpass the Ocelot in size, such as the Leopard, the Lion, and the Tiger, the pupil again assumes a round form.

(2378). The eyes of Mammalia are lodged in bony orbits, as in the oviparous Vertebrata, and in like manner are supported in their movements by a quantity of semifluid fat, with which the orbital cavities are filled up. In Man, as in Birds, Reptiles, and Pishes, six muscles are appropriated to the movements of each eyeball, viz. four recti and two obliqui. The four recti muscles have the same disposition in Mammalia as in Birds; that is, they arise from the margin of the optic foramen, and run forward to be inserted opposite to each other upon the superior, inferior, and lateral surfaces of the sclerotic coat. The inferior oblique likewise offers a similar arrangement in all the Vertebrata, arising from the margin of the internal wall of the orbit, and running outwards to be attached to the external surface of the globe of the eye. But the superior oblique, in the class before us, takes a very peculiar course. Arising like the rest, it passes forward to the upper and inner margin of the orbit, where its tendon is reflected over a little cartilaginous pulley (fig. 410, c), and turns back again to be inserted into the external and posterior aspect of the eyeball.

Structure of the eye.

Fig. 409. Structure of the eye.

Muscles of the eyeball.

Fig. 410. Muscles of the eyeball.

(2379). In addition to the six muscles appointed for the movements of the eye in Man and the Quadrumana, other Mammalia have a seventh, called the choanoid or funnel-shaped muscle. This likewise arises from the borders of the optic foramen, and, gradually expanding, forms a hollow cone interposed between the recti muscles and the optic nerve, the base of the cone being attached to the sclerotic behind the insertion of the recti. Frequently, indeed, this choanoid or suspensory muscle is divided into four portions, in which case the animals so provided would seem to have eight recti muscles.

(2380). The eyelids of Mammalia resemble the human in every respect, excepting that in the lower orders a remnant of the nictitating membrane is still met with; but it is of small dimensions, and unprovided with muscles.

(2381). The lacrymal apparatus exists in all quadrupeds; and the lacrymal gland occupies the same situation as in Man, the tears being poured on to the conjunctiva near the external canthus of the eyelids. The lacrymal ducts, likewise, whereby the tears are conveyed into the nose, so nearly resemble the human as to require no particular description. The caruneulce lacrymales are also met with at the inner canthus of the eyelids. In some quadrupeds, indeed, an additional gland exists, called the glandula Harderi; this is situated behind the internal angle of the eye, and secretes a lubricating fluid, that is discharged beneath the rudiment of the third or nictitating eyelid.

(2382). In Whales, as might be expected from their aquatic habits, no vestige of a lacrymal apparatus is to be seen.

(2383). Behind the optic lobes of the encephalon the nervous centres, from whence the other cerebral nerves take their origin, are so intimately blended together that the anatomist is no longer able to distinguish them from each other. They form, in fact, the "medulla oblongata" and are the commencement of that long series of sentient and of motor ganglia that forms the spinal cord.