The essentials of a sense organ are well shown in the rudimentary forms of a nervous system, where there exists (1) one cell whose business it is to receive the impression, and (2) a nerve thread to carry the impression to (3) a cell whose business it is to receive the impression and take knowledge of it in some way or other. These three elements are necessary for a sensation.
(1) There is a special structure adapted for being affected by a particular kind of influence. Thus the eye is an organ specially formed for being stimulated by the action of light, while the ear is uninfluenced by light but is stimu lated by the waves of sound, and so on. The special structures are called terminal organs.
From the special structure, whatever it may be, (2) a nerve proceeds which is in direct com munication with (3) Nerve-cells in the brain in the region of consciousness. This last is important to notice. The nerve-centre to which the impression, made on the terminal organ, is conveyed by the nerve, must be situated in the brain, if the impression is to give rise to a sensation.
Suppose the impression for some reason or other is arrested in the spinal cord, no sensa tion will result. Thus the nervous chain neces sary for a sensation is not identical with that described on p. 132 as necessary for a reflex action. A man who has had his spine injured, and is thus paralysed iu both legs, does not feel a severe pinch of the skin of either leg, but probably the pinch causes the leg to be spas. modically jerked. An impression has been made on the skin, which has been transmitted to a nerve-centre. But the nerve-centre is in the cord. The injury to the spine has pre vented the impression being conveyed up the cord to higher centres in the brain. Thus, though the impression has been quite sufficient so to stimulate a nerve-centre in the cord as to produce a marked reflex act, the man has been unaware of the pinch ; he has had no sensation. A sensation, then, cannot be produced unless the influence has been transmitted to a higher centre in the brain, and has there excited a change of which the individual becomes aware. An impression may be duly made upon a ter minal organ, but it cannot properly be called a sensation until the person becomes conscious of it. A sensation may therefore be defined as the
consciousness of an impression.
Now, if this is understood, it is easy to per ceive that a sensation may be abolished in various ways. Take, as an example, the sense of sight. A person may be blind, as we all know, because of some injury or disease of the eye, because, that is, of something wrong with the terminal organ; but that is not the only way blindness may be produced. Another person might have perfect eyes, and yet have no sight. The nerve leading from the eye to the brain, the chain of communication, might be inter fered with, destroyed, for example, by the pressure of a tumour, and, therefore, though light duly fell upon the eye and produced there its wonted effect, no knowledge of it would exist because of the rupture of communication —the impression could not be conveyed to the brain. In a third way a person might be pre vented from seeing. Suppose the eye and its nerve unaffected, but the centre in the brain destroyed by the progress of some disease, the impression duly made on the terminal organ and carried along the nerve would reach a dis organized centre, which could not receive it, and no consciousness of sight would arise.
But there are instances of failure to see, illus trating well the necessity for activity of the centre, not dependent upon any disease. Lift the eyelid of a sleeping man, and hold a lighted candle in front of the eye ; an image of the candle flame is properly formed on the back of the eye, an impression is duly conveyed along the nerve to the centre, but the centre slumbers and there is no conscious sight. A man is walk ing through the streets engrossed in thinking over something; a friend walks straight to wards him, and the person is apparently looking directly at his friend, but does not notice him and would pass on did his friend permit. His friend's face and figure made their usual im pression on the eye, which was properly trans mitted to the conscious centre, but that centre was already occupied, and the impression failed to arouse a consciousness of its presence. The same facts might be illustrated in connection with any other sense, but enough has probably been already said to show what are, in general, the conditions of any sensation.