A perfection of the system is brought about by a superstructure built on essentially the same plan. Figure 8 is a diagram illustrating this. The points _S´_ and _M´_ correspond to the points of the same names in figure 7. But several systems (three in the diagram) like that of figure 7 have been combined by connecting neurons in exactly the same manner in which the combination was effected in figure 7. In this higher system (nerve center, we should call it) the points _S´´´_ and _M´´_ have a significance comparable to that of _S´_ and _M´_. [Illustration: FIG. 7.--DIAGRAM OF NERVOUS ARCHITECTURE: REFLEX ARCHES CONNECTED BY A LOW NERVE CENTER. (From Psychological Review, 15, 1908.)] [Illustration: FIG. 8.--DIAGRAM OF NERVOUS ARCHITECTURE: LOWER NERVE CENTERS CONNECTED BY A HIGHER CENTER. (From Psychological Review, 15, 1908.)] 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. [Illustration: FIG. 9.--DIAGRAM OF NERVOUS ARCHITECTURE: HIGHER NERVE CENTERS CONNECTED BY A STILL HIGHER CENTER.] 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_