The Sense Organs

Every sensation arises from the stimulation of a particular form of nervous mechanism known as a sense organ. In the widest acceptation of the term, a sense organ comprises:

(1) receptive cells, to which the stimulus is applied;

(2) sensory neurons, conveying the impulse towards the cerebro-spinal axis,

(3) one or more secondary relays of sensory neurons, which carry the impulse to a special region of the brain, usually of the cerebral cortex. It is generally believed that the character of the sensation depends entirely on the last of these neurons.

The function of the receptive cells is to pick out from the mass of different stimuli constantly arriving, those to which the specific sense organ is attuned. The recognition of a sensation is an act of consciousness, and is usually made without effort; there are, however, certain forms of sensation, notably in connection with the muscular apparatus, which are not appreciated by those uneducated to analyse them, and may therefore be classified as subconscious.

Two important facts in connection with sensations may be noted here: (a) a sensation is not only perceived but can also be localised, either to a part of the body or to a distant object emitting the stimulus; (b) within certain limits the intensity of the sensation bears a direct proportion to the strength of the stimulus. In experimenting on this, Weber measured the amount of increase of stimulus which produces a just appreciable increase in the sensation.

As a result of his observations he formulated the law that the increase of stimulus required to produce a distinct increase of sensation always bears the same ratio to the whole stimulus. The ratio varies for different sensations, but is constant for any one particular sensation. To take an example; the ratio for the pressure sensation is 1:30, so that if a weight of 30 grams be hung on the finger, no difference will be detected in the sensation until one gram more is added.

Fechner, by making the assumption that the just appreciable differences in sensation are equal in amount, elaborated Weber's
law into the followizig: 'if the stimulus increases by a certain multiple (geometrical progression) the sensation increases by regular additional amounts (arithmetical progression)'; this is known as the psycho-physical law.

In the older classification of sensations five were recognised, viz. sight, hearing, taste, smell, and touch. Methods of analysis have, however, shown the presence of many other forms such as temperature, pressure, pain, and the various static sensations. Several internal sensations, e.g. hunger, thirst, etc., are also recounised, but these are extremely difficult to define. In the preceding sections the pathways of the neurons from the receptive cells to the cerebral cortex have been traced. It remains to describe the receptive portions of the sense organs.