Page:Popular Science Monthly Volume 84.djvu/124

120 organs or receptors are, for the most part, the distal ends of sensory neurones. The central organs or adjusters include the proximal ends of these elements, all the association neurones, and the proximal ends of the efferent neurones. The effectors are not neurones at all, but muscle fibers, gland cells or other types of cells under the control of nerves. Thus the ordinary reflex may be said to involve in sequence the activity of a receptor, adjustor and an effector, to use modern terminology, and these three elements are recognizable in every complete reflex arc.

Our own reflexes are sometimes associated with consciousness and sometimes not. When we pass from a region of dim light to one of bright light the pupils of our eyes contract without our being conscious of the fact. In a similar way, when food is introduced into the digestive tract, a whole succession of reflex movements is called forth without any direct relation to our consciousness. On the other hand, if we burn a finger, it is usually withdrawn with full recognition of the sensation and the response. Thus a reflex may or may not be association with a conscious state.

From this standpoint, what is the condition in the lower animals? Have they nervous systems composed of neurones and exhibiting reflexes which in some instances are associated with consciousness, and in others not? In other words, what have been the steps by which has developed that mechanism which serves us at once as the means of our simplest reflexes and the material basis for our intellectual life?

As an example of the lower animals whose nervous activities are worthy of consideration we may take the earthworm. This animal has at its anterior end a small brain from which a ventral ganglionic chain extends posteriorly through the rest of its body. It possesses sensory neurones which extend from the skin into the central nervous organ and motor neurones reaching from the central organs to the muscles. The central organ itself contains association neurones. Thus the three classes of nervous cells which occur in man are also represented in the earthworm but with this difference. The association neurones, which in man are relatively very numerous, are in the earthworm comparatively few. Otherwise the essential composition of the nervous organs in these two forms has much in common.

Not only is the nervous system of the earthworm composed of elements essentially similar to those of the higher animals, but it exhibits similar functional relations. The earthworm responds to a large range of stimuli by appropriate and characteristic reactions, and its movements justify the conclusion that its reflex arcs, like those of the higher animals, involve receptors, an adjustor, and effectors.

Whether certain of the reflexes of the earthworm are associated with consciousness or not is a question that can not be answered definitely,