The Mechanism of Mental States
The mechanism of mental states–the mental machinery by means of which we feel, think, and will–consists of the brain, nervous system, and the organs of sense. No matter what may be the real nature of mind,–no matter what may be the theory held regarding its activities,–it must be admitted that the mind is dependent upon this mechanism for the manifestation of what we know as mental states. Wonderful as is the mind, it is seen to be dependent upon this physical mechanism for the expression of its activities. And this dependence is not upon the brain alone, but also upon the entire nervous system.
The best authorities agree that the higher and more complex mental states are but an evolution of simple sensation, and that they are dependent upon sensation for their raw material of feeling and thought. Therefore it is proper that we begin by a consideration of the machinery of sensation. This necessitates a previous consideration of the nerves.
The body is traversed by an intricate system of nerves, which has been likened to a great telegraph system. The nerves transmit sensations from the various parts of the body to the great receiving office of the brain. They also serve to transmit the motor impulses from the brain to the various parts of the body, which impulses result in motion of appropriate parts of the body. There are also other nerves with which we have no concern in this book, but which perform certain physiological functions, such as digestion, secretion, excretion, and circulation. Our chief concern, at this point, is with the sensory nerves.
The sensory nerves convey the impressions of the outside world to the brain. The brain is the great central station of the sensory nerves, the latter having countless sending stations in all parts of the body, the “wires” terminating in the skin. When these nervous terminal stations are irritated or excited, they send to the brain messages calling for attention. This is true not only of the nerves of touch or feeling, but also of those concerned with the respective senses of sight, smell, taste, and hearing. In fact, the best authorities hold that all the five senses are but an evolution of the primary sense of touch or feeling.
THE SENSE OF TOUCH.
The nerves of the sense of touch have their ending in the outer covering or skin of the body. They report _contact_ with other physical objects. By means of these reports we are aware not only of contact with the outside object, but also of many facts concerning the nature of that object, as for instance, its degree of hardness, roughness, etc., and its temperature. Some of these nerve ends are very sensitive, as, for example, those of the tip of the tongue and finger ends, while others are comparatively lacking in sensitiveness, as, for illustration, those of the back. Certain of these sensory nerves confine themselves to reporting contact and degrees of pressure, while others concern themselves solely with reporting the degrees of temperature of the objects with which their ends come in contact. Some of the latter respond to the higher degrees of heat, while others respond only to the lower degrees of cold. The nerves of certain parts of the body respond more readily and distinctly to temperature than do those of other parts. To illustrate, the nerves of the cheek are quite responsive to heat impressions.
THE SENSE OF SIGHT.
The nerves of the sense of sight terminate in the complex optical apparatus which in popular terminology is known as “the eye.” What is known as “the retina” is a very sensitive nervous membrane which lines the inner, back part of the eye, and in which the fibers of the optic nerve terminate. The optical instrument of the eye conveys the focused light vibrations to the nerves of the retina, from which the impulse is transmitted to the brain. But, contrary to the popular notion, the nerves of the eye do not gauge distances, nor form inferences of any kind; that is distinctly the work of the mind. The simple office of the optical nerves consists in reporting color and degrees of intensity of the light waves.
THE SENSE OF HEARING.
The nerves of the sense of hearing terminate in the inner part of the ear. The tympanum, or “ear drum,” receives the sound vibrations entering the cavities of the ear, and, intensifying and adapting them, it passes them on to the ends of the auditory nerve in the internal ear, which conveys the sensation to the brain. The auditory nerve reports to the brain the degrees of pitch, intensity, quality, and harmony, respectively, of the sound waves reaching the tympanum. As is well known, there are certain vibrations of sound which are too low for the auditory nerve to register, and others too high for it to record, both classes, however, capable of being recorded by scientific instruments. It is also regarded as certain that some of the lower animals are conscious of sound vibrations which are not registered by the human auditory nerves.
THE SENSE OF SMELL.
The nerves of the sense of smell terminate in the mucous membrane of the nostrils. In order that these nerves report the odor of outside objects, actual contact of minute particles of the object with the mucous membrane of the nostrils is necessary. This is possible only by the passage through the nostrils of air containing these particles; mere nearness to the nostril will not suffice. These particles are for the most part composed of tenuous gases. Certain substances affect the olfactory nerves much more than do others, the difference arising from the chemical composition of the substance. The olfactory nerves convey the report to the brain.
THE SENSE OF TASTE.
The nerves of the sense of taste terminate in the tongue, or rather in the tiny cells of the tongue which are called “taste buds.” Substances taken into the mouth chemically affect these tiny cells, and an impulse is transmitted to the gustatory nerves, which then report the sensation to the brain. The authorities claim that taste sensations may be reduced to five general classes, viz.: sweet, bitter, sour, salty, and “hot.”
There are certain nerve centers having important offices in the production and expression of mental states, located in the skull and in the spinal column–the brain and the spinal cord–which we shall consider in the following chapter.