(1800). The brain of an adult fish occupies but a small portion of the cranial cavity, - the space between the pia mater, which invests the brain, and the dura mater, which lines the skull, being occupied by a loose cellular tissue filled with fluid: there is consequently no serous or arachnoid cavity, such as exists in Man. It has been remarked that the interval between the cranium and the brain is considerably less in young than in mature fishes - a fact which sufficiently proves that in them the brain does not grow in the same proportion as the rest of the body; and, indeed, the size of the brain is nearly equal in individuals of the same species, even although the body of one be twice as large as that of the other*.

Brain and cerebral nerves of the Perch (after Cuvier.)

Fig. 319. Brain and cerebral nerves of the Perch (after Cuvier): a, the cerebellum; b, cerebrum; c, olfactory ganglia; i, bulbous commencement of the olfactory nerve; o o, olfactory nerve, terminating in the nasal capsule; n, optic nerve; p, q, third, fourth, and sixth pairs of nerves, appropriated, as in Man, to the muscles of the eyeball; a, ophthalmic branch of the fifth pair; β, superior maxillary branch of ditto; θ, inferior maxillary branch of ditto; μ, opercular branch; ξ, branch of the fifth pair, mounting upwards to join φ, a branch from the eighth pair, running to supply the dorsal region of the body; s s, auditory nerve; t t' nerves belonging to the eighth pair; w, z, nerves answering to the spinal recurrent.

(1801). In these, the lowest forms of Vertebrata, the brain consists of several masses placed one behind the other, either in pairs or singly; these masses, in fact, may be regarded as so many distinct ganglia, the complexity and perfection of which we must expect to become gradually increased as we proceed upwards towards mammiferous quadrupeds.

(1802). The anterior pair of ganglia (figs. 319, c; 320; 322, a) invariably give origin to the olfactory nerves, and consequently may be justly looked upon as presiding over the sense of smell. These ganglia are, in fact, the representatives of those masses which in Man are erroneously called the "olfactory nerves;" for even in the human subject, although their real nature is obscured by the enormous development of other parts of the encephalon, the so-called nerves are not nerves at all, but really lobes of the brain from which the true nerves emanate.

* Cuv. et Val. op. cit.

(1803). The olfactory nerves of Fishes, derived from the lobes alluded to, vary greatly in composition and proportionate size: sometimes they are quite capillary; sometimes thick, though still simple; occasionally they are double or triple, and in some cases are composed of numerous fibres bound up in fasciculi.

(1804). The organs of smell to which these nerves are destined are of very simple structure: - Two excavations are found near the anterior part of the snout, lined with a delicate pituitary membrane, which is variously folded, in order to increase the extent of the sentient surface (fig. 320); and it may be presumed that, from the number of plicae, which varies amazingly, some estimate maybe formed of the relative perfection of the sense of smell in different genera. Into each olfactory chamber the water is freely admitted by two distinct orifices, while behind the pituitary membrane the olfactory nerve swells out into a ganglion (fig. 322,1), from which nervous fibrils radiate, to be distributed over the plicated lining of the nose (k).

(1805). The second pair of ganglia met with in the brain of a Fish (fig. 322, b) give origin to the optic nerves (2), and may therefore very properly be regarded as representing the tubercula quadrigemina of the mammiferous brain. The nerves of vision derived therefrom have no commissure, and present in many species a peculiar structure which is not a little remarkable, each nerve being composed of a broad band of nervous substance, folded up like a fan, and enclosed in a dense membrane, so that when unfolded it presents the appearance delineated at fig. 321, a.

(1806). The eye itself differs in many points of structure from that of terrestrial Vertebrata, its organization being of course adapted to bring the rays of light to a focus upon the retina in the denser element in which the fish resides; the power of the crystalline lens is therefore increased to the utmost extent, and the antero-posterior diameter of the eyeball necessarily contracted in the same ratio, in order that the retina may be placed exactly in the extremely short focus of the powerful lens.

(1807). The eyes of all the Vertebrata are constructed upon principles essentially similar, and present the same tunics and lenses as are met with in the human eye, and, generally speaking, arranged in the same manner as in Man. It is not our intention, therefore, in the following pages minutely to describe the anatomy of the eye in every class which will come under our notice; but taking the human eye, with the construction of which we presume our readers to be intimately acquainted, as a standard of comparison, point out those modifications of the general type of structure common to this division of animated nature.

Organ of smell in the Skate.

Fig. 320. Organ of smell in the Skate.

(1808). The first thing which strikes the attention of the anatomist, when examining the eye of a fish, is the size of the crystalline lens, and its spherical form. This shape, and the extreme density of texture which the lens exhibits, are, indeed, perfectly indispensable. The aqueous humour, being nearly of the same density as the external element, would have no power in deflecting the rays of light towards a focus, and consequently the aqueous fluid in fishes is barely sufficient in quantity to allow the free suspension of the iris: the vitreous humour, from the same reason, would be scarcely more efficient than the aqueous in changing the course of rays entering the eye, and hence the necessity for that extraordinary magnifying power conferred upon the lens.