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The odorous particles must be in the form of gases, in order to be carried by the air into the olfactory region, and the air must be kept in motion, by sniffing it in and out of the nasal cavity, in order to excite the nerve terminals, which are not influenced by the odors of air absolutely at rest, though it be in contact with the mucous membrane of the olfactory tract.
The extreme delicacy of appreciation of odors by the olfactory nerve terminals is very remarkable. Even in human beings,
Section through the mucous membrane of the nasal fossa in the level of the olfactory
region, a, Epithelial cells and bundles of nerves; 6, Glands separated from each other by bundles of nerves, C. (Cadiat.)
whose sense of smell is but poorly developed when compared with that of animals, an amount of odorous substance can be perceived which the finest chemical tests fail to appreciate. Thus, Valentin has estimated that the two-millionth of a milligram of musk is sufficient to excite the specific energy of a man's olfactory apparatus.
No satisfactory classification of odors has been made out. The common division into agreeable and disagreeable smells, or
scents and stinks, is dissimilar in different individuals, and therefore cannot have a physiological basis.
With smell, as with taste, no degree of intensity of stimulation can be said to produce pain, though disgust, nausea, vomiting, and many other nervous operations, may be induced by various smells. The appetites are either excited or annulled by different excitations of the olfactory nerves.
Next in importance to those impulses which we receive from the skin are those conveyed to the brain from the outer world by the second pair of cranial, or the optic nerves.
The ending of the optic nerve differs from any of those met with in the skin, by being enclosed in a very specially arranged organ-the eyeball-an apparatus for bending the rays of light, so that they exactly reach the delicate sheet of complicated nerve ending which is here spread out. Only the blood and other tissues of the eye come in contact with the endings of the optic nerve, which are thus placed out of the way of ordinary nerve stimulation.
Further, the light, of which the optic nerves convey intelligence to the brain, is not properly a nerve stimulus, being merely the waving of an imponderable medium, the existence of which is assumed. Besides the special arrangements in the eyeball for bringing the
rays of light to bear on the nerve endings, there must here be some extremely sensitive arrangement by which the ether waves, which we call light, can be converted into a nerve stimulus, or in some way made to affect the nerve terminals in the retina.
By means of the sense of sight we obtain knowledge of objects at a distance from us, because all these objects reflect more or less light, and thus make different impressions upon the terminals of the optic nerve forming the outer layer of the retina.
Light, then, is the adequate stimulus for the retinal nerve endings, and the impulse caused by light is the only impression the optic nerve is in the habit of carrying to our sensoria, where the sensation of light is formed and distributed among the cells of the brain, so as to enable us to come to visual judgments and conclusions. As already mentioned, no matter what stimulus electric, mechanical, or other, be applied to the fibres of the optic nerve, the sensation produced is simply light, and this is thought of as if it came through the eye from the outer world.
The study of the function of vision may be divided into :
1. The path the light takes on its way through the eye to reach the retina.
2. The molecular changes in the retina which give rise to stimulation of the optic nerves.
3. The sensations arising in the sensorium as the result of the molecular changes set up in the cerebral nerve cells by the impulses from the optic nerve.
4. The visual perceptions and judgments which our consciousness is capable of elaborating from the visual sensations.
THE TUNICS OF THE EYEBALL. The organ of vision of vertebrate animals is enclosed in a firm case of fibrous tissues called the sclerotic coat, which is continuous with the sheath of the optic nerve. It is seen between the eyelids under the transparent conjunctiva, and known as the white of the eye. It gives shape and protection to the eye, and though translucent, is not transparent. In front, a round, window-like portion, called the cornea, forms the anterior segment of this protecting covering of the eyeball. The cornea is distinguished from the sclerotic not only by its glass-like transparency, but also by being part of a lesser sphere than the sclerotic, and thus projecting a little more than the rest of the bulb.
Closely attached to the inside surface of the sclerotic is a soft, thin, black, vascular sheet of tissue called the choroid coat, which supplies the eyeball with blood. It is made up chiefly of blood vessels and stellate, pigmented, connective tissue cells. Its outer layer is traversed by arteries and veins of relatively large size, and its inner layer is composed of a dense network of closemeshed capillary vessels. As the cornea region is approached, the choroid is peculiarly modified and thrown into folds, called ciliary processes, forming a series of vascular folds, radiating from the margin of the cornea. At the edge of the cornea the choroid is more firmly attached to the sclerotic by a circular muscle (ciliary muscle), and also by bands of tissue from the posterior surface of the cornea, which hold it in position ; the fibres of the ciliary muscle, running under the ciliary processes, radiate from the margin of the cornea toward the choroid, to which they are attached. In a modified form, known as the iris, this vascular and pigmented coat of the eye leaves the sclerotic, and hangs freely in a fluid, being recognized through the clear
Diagram of a horizontal section through the human eye. 1. Cornea ; 1'. Conjunctiva; 2. Sclerotic; 3. Choroid; 4. Ciliary processes; 4'. Ciliary
muscle: 5. Suspensory ligament of lens ; 6. So-called posterior chamber, between the iris and the lens; 7. Tris; 7. Anterior chamber in front of the iris; 8. Optic nerve : 8'. Entrance of central artery of the retina ; 8". Central depression of retina or yellow spot: 9. Anterior limit of the retina ; 10. Canal of Petit in front of the hyaloid membrane ; 11. Aqueous chamber: 12. Crystalline lens ; 13. Vitreous humor : 14. Circular venous sinus which lies around the cornea; a-a, anterior-posterior, and b-b, transverse axis of bulb.
cornea as a colored circular curtain, attached to the inside of the periphery of the cornea, having a central aperture, which looks black, and is familiarly known as the pupil. The pupil is merely an opening in the iris, which allows the rays of light to pass into the interior of the eyeball.