Sensory ectodermic cells are found on the surface of the body in Ichthyopsida as branchial sense organs or organs of the lateral line (infra under Ichthyopsida), and in Pisces of end-buds. End-buds in other Vertebrata are restricted to the oral cavity, and in Mammalia as 'taste' buds to certain papillae or lamellae on the tongue and on the epiglottis.
1 For the difficult questions connected with this subject of segmental cranial nerves, and their connections, the student must consult the original authorities cited below (p. 358).
2 The segmental nature of the olfactory nerve is not universally admitted.
Touch cells, i. e. ganglion cells, terminating a nerve filament, are found in the cutis of Anuran Amphibia and higher forms, often united within a common sheath forming touch corpuscles. Finally, in Sauropsida and Mammalia Pacinian corpuscles or terminal nerve filaments surrounded by greatly developed sheaths are found in the skin, among the muscles, etc. The olfactory mucous membrane in some Pisces, e.g. Flying Fish, Pike, and Urodele Amphibia, e. g. Newt, appears to consist of end-buds intermingled with non-sensory epithelium, but in most Vertebrata it consists of sensory epithelium mingled with supporting cells. The former are terminated by sensory hairs, except in Pisces and Mammalia. The nose is originally a depression, but becomes in the Dipnoi and higher forms a sac, with an external opening in the face and an internal opening into the mouth. A portion of the nasal cavity becomes separated off from the nose proper. It is known as Jacobson's organ, and is supplied by the fifth nerve as well as by the olfactory. It exists in many Lacertilia, Ophidia, and Mammalia, and is represented in Amphibia. The retina of the eye, the retinal epithelium and optic nerve are derived from a vesicle of the brain, originating from the thalamencephalon, i. e. secondarily from the epiblast.
The retina is composed, proceeding from within outwards, of an inner layer of nerve-fibres derived from the optic nerve; a layer of ganglion cells; an inner granular, or cell layer; and an outer granular, or cell layer - the cells of which give origin to the visual rods, or rods and cones. The outer free ends of the latter are immersed in a layer of pigment cells, also derived from the optic vesicle. The retinal elements are supported by connective tissue. Capillary blood-vessels enter the retina in Mammalia, Chelonia, and the Eel. There is a spot lying in the axis of vision known as fovea centralis, or in Primates among Mammalia from its colour as macula lutea, in which the cones with their cells are alone present, the other layers thinning away. Between the retina and the lens intervenes a jelly-like vitreous humour, into which projects in many Pisces, Reptilia, and in Aves a pigmented vascular process - the pecten, marsupium or processus falciformis. The lens is cellular, and is formed by an involution of the epiblast. There is an adjustable fibro-muscular diaphragm, or iris, placed in front of it.
The eye is enveloped in two protective coats, an inner vascular pigmented choroid, and an outer fibroid or cartilaginous sclerotic, continuous in front with a transparent cornea, which is in part an epiblastic, in part a mesoblastic, structure. The eye-ball is moved by four recti and two oblique muscles. It is usually protected by an upper and lower eyelid, and, in very many instances, by a third lid or nictitating membrane, which has frequently special muscles of its own. Two glands, a lacrymal in connection with the upper lid, a Harderian with the third lid, are commonly present; and their secretion is carried to the nasal cavity by a special lacrymal duct which arises at the anterior or inner angle of the eye. The auditory organ commences as an epiblastic involution. It becomes saccular, but the pedicle of invagination persists as the aquaeductus vestibuli, or saccus endolymphaticus, which in some instances, Lacertilia especially, becomes much enlarged, and in many Elasmobranchii remains open externally. The ear-sac differentiates into two connected vesicles, the utricle and saccule. Three semicircular canals, anterior vertical, posterior vertical, and external horizontal, originate from the utricle; a lagena, or cochlea, from the saccule. The whole constitutes the membranous labyrinth.
The auditory nerve is distributed to certain spots, the maculae of the utricle, saccule, and cochlea, the cristae acusticae of the semicircular canals, the epithelium of which consists of sense-cells and supporting cells. The whole apparatus is filled with a liquid, or endolymph. Calcareous otoliths are found in the sensory region of the maculae, except in the cochlea of Mammalia. They vary in size and are borne by the sense hairs of the epithelium, and when small are united by a slimy substance. The membranous labyrinth is inclosed in the auditory capsule, but is separated from it by a space filled with perilymph. In Pisces this space opens into the arachnoid space of the brain by passages round the auditory nerve and the aquaeductus vestibuli, but by a special duct the aquaeductus cochleae (ductus peri-lymphaticus) in Amphibia and higher Vertebrata. Two membranous spots, the fenestra ovalis and fenestra rotunda, occur on the outer wall of the auditory capsule, where it bounds the perilymphatic space.
They are found in most Amphibia and all higher Vertebrata. To the first is attached either a single auditory ossicle, or a chain of ossicles, which connect it to the tympanic membrane (infra). The single ossicle is the stapes ( = hyomandibular of Pisces), found in Amphibia and Sauropsida, where it is often broken up into a series of segments (supra, p. 339), and is generally known as columella auris. The chain of ossicles is found in Mammalia, and comprises a stapes, articulated to an incus (= quadrate of Sauropsida, &c), and that in turn to a malleus (=articular part of Meckel's arch). These bones are contained in Anuran Amphibia and higher Vertebrata in a tympanic cavity, produced by an outgrowth from the pharynx in the position of the first visceral cleft or spiracle, which atrophies almost completely. The part opening into the pharynx is the Eustachian tube. The outer wall of the outgrowth, together with mesoblast and epiblast, forms the tympanic membrane to which one end of the columella auris, or in Mammalia the malleus, is attached.