The sensory system informs areas of the cerebral cortex of changes that are taking place within the body or in the external environment. The special sensory receptors respond to special individual stimuli such as sound waves, light, taste, smell, pressure, heat, cold, pain, or touch. Positional changes, balance, hunger, and thirst sensations are also detected and passed on to the brain.

SMELL
Odor is perceived upon stimulation of the receptor cells in the olfactory membrane of the nose. The olfactory receptors are very sensitive, but they are easily fatigued. This tendency explains why odors that are initially very noticeable are not sensed after a short time. Smell is not as well developed in man as it is in other mammals.

TASTE
The taste buds are located in the tongue. The sensation of taste is limited to sour, sweet, bitter, and salty. Many foods and drinks tasted are actually smelled, and their taste depends upon their odor. (This interdependence between taste and smell can be demonstrated by pinching the nose shut when eating onions.) Sight can also affect taste. Several drops of green food coloring in a glass of milk will make it all but unpalatable, even though the true taste has not been affected.

SIGHT
The eye, the organ of sight, is a specialized structure for the reception of light. It is assisted in its function by accessory structures, such as the eye brows, eyelashes, eyelids, and lacrimal apparatus. The lacrimal apparatus consists of structures that produce tears and drains them from the surface of the eyeball.

Structure of the Eye
Approximately five-sixths of the eyeball lies recessed in the orbit, protected by a bony socket. Only the small anterior surface of the eyeball is exposed. The eye is not a solid sphere but contains a large interior cavity that is divided into two cavities, anterior and posterior. The anterior cavity is further subdivided into anterior and posterior chambers (fig. 1-48).

The anterior cavity of the eye lies in front of the lens. The anterior chamber of the anterior cavity is the space anterior to the iris, but posterior to the cornea. The posterior chamber of the anterior cavity consists of a small space directly posterior to the iris, but anterior to the lens. Both chambers of the anterior cavity are filled with a clear, watery fluid called aqueous humor. Aqueous humor helps to give the cornea its curved shape.

The posterior cavity of the eye is larger than the anterior cavity, since it occupies all the space posterior to the lens, suspensory ligaments, and ciliary body. The posterior cavity contains a substance, with the consistency similar to soft gelatin, called vitreous humor. Vitreous humor helps maintain sufficient pressure inside the eye to prevent the eyeball from collapsing.

The eyeball is composed of three layers. From the outside in, they are the sclera, choroid, and retina (fig. 1-48).

OUTER LAYER.-The outer layer of the eye is called the sclera. The sclera is the tough, fibrous, protective portion of the globe, commonly called the white of the eye. Anteriorly, the outer layer is transparent and is called the cornea, or the window of the eye. It permits light to enter the globe. The exposed sclera is covered with a mucous membrane, the conjunctiva, which is a continuation of the inner lining style="mso-spacerun: yes"> of the eyelids. The lacrimal gland produces tears that constantly wash the front part of the eye and the conjunctiva. The tear gland secretions that do not

Figure 1-48.-Transverse section of the eye.

evaporate flow toward the inner angle of the eye, where they drain down ducts into the nose.

MIDDLE LAYER.-The middle layer of the eye is called the choroid. This layer is a highly vascular, pigmented tissue that provides nourishment to the inner structures. Continuous with the choroid is the ciliary body. The ciliary body is formed by a thickening of the choroid and fits like a collar into the area between the retina and iris. Attached to the ciliary the body are the suspensory ligaments, which blend with the elastic capsule of the lens and holds it in place.

Iris.-The iris is continuous with the ciliary body. The iris is a circular, pigmented muscular structure that gives color to the eye. The iris separates the anterior cavity into anterior and posterior chambers. The opening in the iris is called the pupil (fig. 1-49). The amount of light entering the pupil is regulated through the constriction of radial and circular muscles in the iris. When strong light is flashed into the eye, the circular muscle fibers of the iris contract, reducing the size of the pupil. If the light is dim, the pupil dilates to allow as much of the light in as possible. The size and reaction of the pupils of the eyes are an important diagnostic tool.

Lens.-The lens is a transparent, biconvex (having two convex surfaces) structure suspended directly behind the iris. The optic globe posterior to the lens is filled with a jellylike substance called vitreous humor, which helps to maintain the shape of the eyeball by maintaining intraocular pressure. The lens separates the eye into anterior and posterior cavities.

INNER LAYER.-The inner layer of the eye is called the retina (fig. 1-48). It contains layers of the nerve cells, rods, and cones that are the receptors of the sense of vision. The retina is continuous with the optic nerve, which enters the back of the globe and carries visual impulses received by the rods and cones to the brain. The area where the optic nerve enters the eyeball contains no rods and cones and is called the optic disc (blind spot) (fig. 1-50).

Rods.-Rods respond to low intensities of light and are responsible for night vision. They are located in all areas of the retina, except in the small depression called the fovea centralis, where light entering the eye is focused, and which has the clearest vision.

Cones.-Cones require higher light intensities for stimulation and are most densely concentrated in the fovea centralis. The cones are responsible for daytime vision.