ASSOCIATION TRACTS.
When discussing cerebral localization, it was pointed out, that the various divisions of the brain were divided in a general way, according to their function, into higher and lower parts. Functionally the highest division is the cerebrum, with the cerebral cortex; the lower divisions intervene between the spinal cord and the cerebrum and include the medulla oblongata, the pons and the cerebellum, the mid-brain and the diencephalon or inter-brain.
All nerve tracts, which convey to the central nervous system the most varied impulses from the individual sense-organs and the various organs within the body, find their immediate ending within the lower brain-centres; within these lower centres arise efferent paths, by means of which the stimuli received are again projected towards the periphery and transferred to the organs of movement. In this manner are brought about all those movements that we designate as simple and complex reflexes, which occur without participation of our consciousness. The impulses conveyed to the central nervous system, however, are not confined to the subcortical centres, but are carried by other paths to the cerebral cortex, where, in the apprdpriate sensory centres, impulses are called forth which psychically correspond to what we designate as sensation. This impulse within the sensory cortical centres continues, so long as the stimulus continues. With the stimulus, the impulse disappears and therewith the sensation also ceases. We are able, however, to picture an object, even when we no longer perceive it, or to recognize it when it again appears. Therefore, on its first appearance, the stimulus must have called forth a permanent impulse, in addition to the vanishing sensory impulse; the latter is designated the concept impulse. The retention of this impulse makes possible the recognition, the proving, or the representation of the object; that is, there remain per sistent traces of previous sensory or motor impulses, the so-called latent dispositions. These latent dispositions or subconscious impressions, when later awakened by new impulses, render possible the conscious memory or conception of sensation and movement. The ability to call into activity and to convert the latent dispositions or impressions into conceptions is what we call thought.
In addition to this mnemonic function, the cerebrum possesses the associative function. One conception can awaken others by reason of the linking together of the latent dis positions. By union of partial conceptions (visual, gustatory, olfactory, tactile and other
sensations), the complete conception is attained; by the blending of the complete con ceptions, the general conceptions are formed. In this way are " reproduced " entire complexes of conceptions, which are definitely connected and, as it were, lie prepared; it may be, however, that certain complexes of conception are arranged in other and new sequences, new conceptions being thereby "produced." The associative function consists, therefore, in the reproduction and production of conceptions, and on this possibility of a definite sequence of conceptions depends the exercise of the higher psychic processes, that is, thought.
By means of these associative processes, the individual cortical areas within the same projection and memory fields, as well as the different projection and memory fields, are brought into connection with one another. Such connection between the dispositions or residues of the same kind exists everywhere within the corresponding cortical areas. The association between residues of different kinds, as well as the connection of projection areas with memory centres and of the various projection and memory centres with one another, is established by means of the association fibres, which as short and long fibres unite adjoining convolutions and remote regions respectively.
Since, however, the widely different processes of the outer world and of the body proper give rise to the formation of manifold impressions and to the exercise of the most simple as well as the highest psychic processes, something further always occurs. The influences taken up by the organism react outwardly, since they always find expres sion in the various movements of the organs. While the purely reflex reactions are carried out unconsciously, through the agency of the lower brain-centres and without the participation of the cerebrum, the voluntary movements, our conduct and voluntary acts are dependent upon the activity of the cerebral cortex, every action, indeed, being determined by conceptions and, in the final analysis, by kinaesthetic or motor concepts. These relations will be best understood, if, in conclusion, we consider more closely those most important movements concerned in speech, by means of which our entire sensations, conceptions and thoughts find expression.