The next thing which we have to consider is how knowledge of the external world reaches the central nervous system. Sensations of touch, temperature, and pain are fairly easy to understand, since the nerves which convey such impressions have numerous endings in the skin. End organs of nerves in the joints and muscles doubtless enable the animal to perceive and estimate strain and resistance in moving or lifting things. But the power of perceiving the chemical peculiarities of things; light, involving the formation of visual images, which we call seeing; sound; and position and equilibrium, it is not possible for the whole surface of the body to possess. The principle of division of labour is extended to the task of perception as well as to that of motion; and cells, with their property of responding to light, vibration, chemical stimulation, etc., are grouped together to form special organs, connected with the central nervous system by special nerves.
Perhaps the most important factor which can influence protoplasm is the chemical nature of its surroundings; and in [the first essay], on the general nature of protoplasm, we touched upon the way in which it is drawn towards some substances, and repelled by others.
In the body there are two sets of cells deputed to act for the rest in this particular. One set is situated in the membrane lining the nose, over which the air we breathe passes; and these cells examine our gaseous surroundings, and warn us, by what we term ‘smell,’ whether the atmosphere is fit for us or we had better seek a purer. The other set is for the examination of liquids. Against these we are protected by our skin, and, as we do not absorb anything through it, it is devoid of the power of examining the things it touches. But with our food it is different; we must have the power of testing that. Accordingly, there are Customs officers in our mouth in the form of little groups of cells, which report upon the liquids and solids moistened by saliva, and enable the animal to reject pernicious imports. Thus, the stimulation of a small portion of the protoplasm composing a body is transmitted over the whole, and is able to awake in it the necessary response.
Diagram 51.
Diagram 52.
Diagram 53.
