Causes of Deafness

A few people are born deaf; many more become deaf as a result of accident, infection, or aging. As one grows older, the eardrum tends to become thicker and less flexible in the transmission of sound waves. In some persons, also, irrespective of age, there is an increase in bony tissue so that the stirrup bone becomes firmly fixed in the oval window and is no longer able to function in the transmission of sound. This condition is called otoscle­rosis. In some such cases, carefully selected, deafness can be alleviated by a surgical oper­ation   (fenestration   operation)   in   which a not so important to his existence as they are to most lower animals. Both senses may warn of danger, as in smelling smoke or biting into some foodstuff that just doesn't "taste right."

The sensory end-organs for smell (olfactory chemoreceptors to give them their technical name) are located at the top of the inner sur­face of the nose. These olfactory cells are imbedded in mucous membrane; from them other nerve fibers pass up to the olfactory lobe in the brain, where the smell is "interpreted." The olfactory cells pick up odors from mole­cules or small particles released by various substances. These particles are present in the air we inhale, and when they come in contact with the olfactory cells, they stimulate the appropriate end-organs.

Human beings can distinguish about 60.000 odors; but these are combinations of a rela­tively few specific classes of odors to which the olfactory cells directly respond. According to one classification, these cells respond to only seven odors, described as burnt, acid, putrid, spicy, fragrant, fruity, and resinous.

Taste is the sense by which we perceive the flavor of substances placed in the mouth. The end-organ receptors through which this is accomplished are called "taste buds." The im­pulses from these receptors are, of course, conveyed to the brain, where they register as taste sensations. For practical purposes the taste buds may be considered to be located on the tongue, though there are a few scattered in the pharynx.

Human beings can actually distinguish only four different tastes: sweet and salt in the buds at the tip of the tongue; sour (or acid) along the sides of the tongue; and bitter at the back of the tongue. The taste buds are react­ing to chemicals in solution. The combina­tions of the four basic taste sensations provide a wide variety of flavors. Whether a food tastes "good" or "bad" to an individual is not based on the intrinsic nature of the foodstuff but rather on his past (learned) experiences with the particular food or flavor.

Stimulation of the taste buds brings about an important unconscious reflex reaction. It induces the secretion of salvia in the mouth and gastric juices in the stomach, thus aiding the process of digestion.

While the nose and tongue, as we have noted, are sense organs, they also serve other functions. The nose and paranasal sinuses are important adjuncts to the respiratory system. The tongue, used for mixing foods in the mouth and for swallowing, assists the diges­tive system. Both tongue and nose are in­volved in the process of human speech.