The Olfactory Organs
As the term nasal organ is ambiguous, commonly referring to the nose, we propose to use the term olfactory organs when referring to the sense organ of smell. The nose is merely the wall of the two nasal passages leading to the olfactory organs. The sense cells of these organs work in a similar fashion in all vertebrates, being stimulated by substances which dissolve in their moist surface, so that smell is essentially a chemical sense. We naturally lose the sense of smell when this stimulation is prevented, e.g. when the surface dries, as on a hot, dusty day, or when the substances in solution are prevented from reaching the sense cells by a layer of viscous fluid, as when we have a cold. In order to trap these substances from the air, the sense cells are spread over a large area on a framework of cartilage or bone arranged in a series of either flat plates or scrolls. Fish and Amphibia commonly possess valves to the external nostrils to prevent the entry of muddy water. Mammals have hair in the nasal passages to filter out dust and larger particles before the air reaches the olfactory organs. The olfactory surfaces also possess cells for producing mucus to moisten the surface. These cells work overtime when stimulated by germs of the common cold, so that the germs may be washed away as the liquid drains from the olfactory organs. The olfactory lobes of the brain are separated from the olfactory organs only by a thin sheet of bone which is perforated by numerous holes through which the fibres of the olfactory nerves pass. When the olfactory organs are stimulated, impulses pass along these fibres to the brain, eventually reaching a region of it where the sensation is experienced, I.e. the sensation of smell is in the brain, not in the olfactory organs which merely ’pick up ’the stimulus.
The Taste Organs
The tongue is the organ of taste. Situated at various places on its surface are smallof sensory cells known as ’taste-buds. ’ These are connected with branches of either the seventh or ninth nerves. These sensory cells respond to one or other of the main sensations : sweet, bitter, sour, salty, alkaline and metallic. Sweetness and saltiness are perceived by the taste-buds at the front of the tongue ; bitterness by those at the back and sourness by those at the sides. This can readily be ascertained by touching these different areas with glass rods previously dipped respectively in solutions of sugar, salt, quinine and alum.
All vertebrates have their eyes built upon the same plan. Each eyeball is spherical in shape, the part normally exposed being the cornea. It lies in a socket to which it is attached by six muscles which move it up and down, from side to side, or in an oblique movement. The skin of the animal is continued over the eyeball as a tough transparent conjunctiva. The eyeball is hollow, being filled with transparent humour and possessed of three layers to its wall. The outermost, the sclerotic, is of very tough connective tissue, forming the ’white ’of the eye where exposed in front. The middle layer is a pigmental choroid with numerous capillaries, this being the nutritive layer of the eye. Lining this is a very delicate 400 sheet of tissue, the retina, in which are the cells sensitive to light, the yellow spot being an area where the sense cells are most concentrated. This layer, which is also pigmented on its inner side, is well supplied with the fibres and dendrites on the second nerve which penetrates right through its inner surface at the blind spot where there are therefore no sense cells and which is a region of the retina insensitive to light. Each layer is modified in front: the cornea being the transparent portion of the sclerotic; the choroid being continued in front as a pigmented muscular curtain, the iris, in which is an aperture, the pupil ceases altogether, in its place there being a circular converging transparent lens held in place by connective tissue forming the suspensory ligament. The suspensory ligament at its outer rim is attached to projections of the choroid known as the ciliary processes. In front of the lens, filling the anterior chamber, the humour is watery ; behind it, it is somewhat jelly-like. Both kinds of humour are under pressure and thereby keep the eyeball in shape, for although tough no part of the wall is rigid.