Several of these larger systems (three in the diagram) are combined again by means of connecting neurons in exactly the same manner as before. This is illustrated by figure 9. The points S´´´ and M´´´ have a significance like that of S´ and M´, S´´´ being nearer to sensory points of the body than to motor points, M´´´ being nearer to motor points. This system of connecting neurons represents again what we may call a higher nerve center—higher still than those which are combined in it.
Thus we may conceive any number of systems, one still higher than the other. And we may understand how it is possible that simpler mental functions may enter into a combination, forming a unitary new function, without completely losing their individuality as functions of a lower order; for combinations of simple functions represented by direct connections into complex functions are brought about only by mediation of higher connecting neurons which represent the less direct connections of sensory and motor points. The most manifold associations are made possible. A practically inexhaustible number of different adaptations is structurally prepared, so that the most complicated circumstances and situations find the organism capable of meeting them in a useful reaction. This type of nervous system is the property of the highest animals and of man. The lower type of nervous system is represented by the reflex arches of the so-called spinal and subcortical centers. The higher type is represented by the cerebrum and cerebellum, which during a process of evolution covering hundreds of thousands of years have gradually been developed to serve as the highest centers of the nervous system.
[3.] The Anatomy of the Nervous System
The most prominent part of the nervous system is that inclosed within the skull and the vertebral column. The spinal cord runs all through this column up to the skull. Entering into the skull, it thickens and forms what is called the bulb (medulla oblongata). It then divides into several bodies, which are referred to as the subcortical centers, because they are located below the cortex, which is the surface layer of the cerebrum, or large brain. These subcortical centers contain the central ends of neurons which are links of chains of afferent neurons coming from the higher sense organs and from the sensory points of the skin and the internal organs. Chains of efferent neurons, on the other hand, take their origin in the subcortical centers, reaching at their peripheral ends the motor points of the body, that is, the muscle fibers of our skeletal muscles and of the muscle tissues contained in the alimentary canal and the other internal organs.