(1593). The posterior portion of the orbital capsule is occupied by a large cavity quite distinct from the globe of the eye, although its walls are derivations from the sclerotic tunic, wherein is lodged the great ganglion of the optic nerve (k), imbedded in a mass of soft white substance. This supplementary chamber is formed by a separation of the sclerotic into two layers, of which one, already described (i), forms the posterior boundary of the eyeball, while the other (h), passing backwards, circumscribes the cavity in question. On entering the compartment thus formed, the optic nerve (q) dilates into a large reniform ganglion (k), almost equal in size to the brain itself; and from the periphery of the optic ganglion arise the numerous nervous filaments which, after perforating the posterior part of the globe of the eye, expand into the post-pigmental retina.

* Cyclopaedia of Anatomy and Physiology, art. Cephalopoda.

(1594). Between the globe of the eye (g) and the cornea (f) is a capacious serous cavity, which extends to a considerable distance towards the posterior part of the orbital chamber, and holds the same relation to the visual apparatus, and the cavity in which it is lodged, as the serous lining of the human pericardium does to the heart, and the fibrous capsule in which that viscus is lodged, - evidently forming an arrangement for facilitating the movements of the eye. The serous membrane which lines this cavity, after investing the inner surface of the cornea and the interior of the orbit, is reflected upon the outer surface of the sclerotic tunic of the eye, which it likewise covers, and moreover, at the front of the eyeball, enters the aperture which in the eye of a vertebrate animal would be occupied by the cornea, lines the chamber corresponding with that of the aqueous humour, and even passes over the anterior surface of the crystalline. This serous membrane Cuvier, very improperly, named the "conjunctiva"; but, as Professor Owen has suggested*, it is evidently rather analogous to the membrane of the aqueous humour, here excessively developed in consequence of the want of a cornea in the sclerotic aperture.

This serous cavity is not, however, a completely-closed sac, but, as is frequently the case with the serous membranes of fishes and reptiles, is in communication with the surrounding medium, through the intervention of a minute orifice visible in the transparent tegumentary cornea.

Loligopsis Verani.

Fig. 293. Loligopsis Verani.

(1595). Four muscular slips are appropriated for the movements of this remarkable eye, and serve to direct the axis of the organ so as to ensure distinct vision; they arise principally from the orbital prolongations of the cranial cartilage, and are inserted into the sclerotic tunic.

(1596). It is always interesting to the physiologist to observe the earliest appearance of a new system of organs, and witness the gradual development of additional parts, becoming more and more complicated as we advance from humbler to more elevated grades of the animal creation. The progressive steps by which the auditory apparatus of the Vertebrata attains to that elaborate organization met with in the structure of the human ear are not a little curious. In the simplest aquatic forms the central portion of the internal ear alone exists, imbedded in the as yet cartilaginous cranium. Gradually, as in fishes, semicircular canals, prolonged from the central part, increase the auditory surface, but still have no communication with the exterior of the body. In reptiles and birds, destined to perceive sonorous impressions in an aerial medium, a tympanic cavity and drum are superadded; and lastly, in the Mammiferous orders, external appendages for collecting and conveying sound to the parts within, complete the most complex and perfect form of the acoustic instrument.

(1597). As far as is yet known, the Tetrabranchiate Cephalopods have no distinct organ of hearing; but in the Dibranchiata, an ear, lodged in an internal cranium, for the first time presents itself to our notice, and at the same time exhibits the lowest possible condition of a localized apparatus adapted to receive sounds.

(1598). In the anterior and broadest part of the cartilaginous cranium1, where its walls are thickest and most dense, are excavated two nearly spherical cavities (fig. 294, d), which in themselves represent the osseous labyrinth of the ears. A vesicle or membranous sacculus (c), likewise nearly of a spherical form, is suspended in the centre of each of these cartilaginous cells by a great number of filaments, that are probably minute vessels. The two auditory nerves derived from the ence-phalon enter these cavities through special canals; and each, dividing into two or three branches, spreads out over the vesicle to which it is destined. The auditory vesicle itself is filled with a transparent glairy fluid, and contains, attached to its posterior part, a minute otolith (1, 2, 3), of variable shape in different genera, the oscillations of which doubtless increase the impulses whereupon the production of sound depends.

* Cyclopaedia of Anatomy and Physiology, he. cit. p. 552. 1 Cuvier, Memoire sur la Poulpe, p. 41.

Brain and auditory apparatus of the Cuttle fish: a, b, brain; c, auditory apparatus.

Fig. 294. Brain and auditory apparatus of the Cuttle-fish: a, b, brain; c, auditory apparatus; d, cavity in which it is lodged; e,f, g, the eye. 1, 2, 3, otolith.

Generative organs of the female Cuttle fish. (After Curier.)

Fig. 295. Generative organs of the female Cuttle-fish. (After Curier).

(1599). Such is the simplest form of an ear; and if the reader will compare the organ above described with that possessed by the highest Articulata, as, for example, the Lobster (§ 1047), the similarity of the arrangement will be at once manifest.