large amount can create an impulse wave train that moderates through the propagation of an expanding, spherical wave front in the atmosphere; hence the thunder follows the lightning. We can postulate an answer to the age-old question, “If a tree falls in the forest with no one around to hear it, does it make a sound?” The answer is, “Yes, because people have ears.” The sound existed before the ears did; the ears exist because the sound already did.

Human hearing is a rather complex example of a mechanical to electro-chemical transducer; complex at least with respect to other mechanical assemblies found within the human body. The sense of hearing encompasses the three divisions of the ear system: the outer ear, the middle ear and the inner ear. Sound waves are first detected and collected by the mechanical assembly of the outer ear. This assembly includes the fleshy, external ear (the auricle) located on the side of the head. The assembly acts as an acoustic wave-guide to focus externally generated sound waves (i.e. pressure waves in the atmosphere) into the ear canal where they encounter the tympanic membrane, more commonly called the eardrum that forms the boundary of the middle ear. The eardrum is connected via a fluid filled channel to an assembly of very small bones whose movement is modulated by the membrane’s movements. Deep within the inner ear, the vibrations propagated through a fluid filled passage finally encounter a collection of hair cells that convert the mechanical vibration into a nerve impulse that is then conveyed to the brain.

The sense of touch is a very short-range facility; to be activated there must be physical contact between the impinging object and the human body. In our previous considerations of the basic forces through which physical entities interact, we alluded to the fact that physical contact in fact derives from the electromagnetic force. Two entities never actually achieve contact in the sense that a part of an atom from one physical entity directly coincides in space with a part of an atom from a different entity. Rather, the atoms which make up the different entities come in close enough proximity that the electromagnetic forces derived from the electron cloud of the atoms of one entity strongly interacts with, and actually repel, the electron cloud of the atoms of the other entity. If one entity is driven by its skeletal motor system toward a different entity, then the strength of this electromagnetic repulsion is converted through nerve endings into impulses that make their way to the brain. The result, insofar as the brain is concerned, is the recognition of a sensation of touch.

Earlier in this chapter we commented briefly on the three macroscopically observable mechanisms for physical interactions based on the electromagnetic force: conduction, convection and radiation. Two of the basic human senses, those of smell and of taste, make use of chemical reactions in their respective sensors. Consequently, these senses actually make use of the conduction mechanism. The odor of some object derives from the emission of particles from the object that are conveyed to the olfactory sensory organs within the nasal passages of a person. These particles are typically molecular emissions from the source object. Once conveyed into the nasal passages, these molecular components encounter organic olfactory sensors with a material on their surface into which the components dissolve.

The sense of smell is important to the human species, but is a bit less developed than is the case for other mammals. For example, the human nose contains approximately 40 million olfactory sensors, compared to approximately 100 million for rabbits and 1 billion for dogs. As one might expect from these numbers, the sense of smell is far more important to dogs as they cope with their physical environment than is the case for humans. It’s interesting to note the comparison between rabbits and dogs, since one might assume the rabbit to be prey for the dog. Were it not for hearing, as reflected in the relative size of the ears, rabbits might be at a distinct disadvantage. Actually, they probably are; it is just that rabbits compensate by reproducing like, well, rabbits.

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