Caudal and cephalic influences of the brain stem reticular formation.

HEN THE HEAD END of the central nervous system became preoccupied in phylogeny with behavior of the individual as a whole, as contrasted with that within component segments, the cerebral hemispheres and cerebellum were appended to the outside of the cranial neuraxis and, at the same time, a considerable volume of neural tissue was added to its interior, in what is known as the reticular formation of the brain stem. In many respects this brain stem reticular formation appears not so much to have been a completely new development as a rostra1 enlargement and specialization of interneuron collections present throughout the intermediate grey of the cord. Both bear the same topographical relation to the central canal and are so interposed between definitely afferent and efferent neural elements. Ascending and descending propriospinal connections from spinal interneurons, serving intersegmental activity within the cord, similarly are prototypes of comparable connections of the brain stem reticular formation, which enable it to influence activity at caudal and cephalic neural levels. By means of its descending connections (2, 83, 84, 6, 15, 16, 79, 80), the reticular formation of the brain stem is capable of modifying activity within the cord, either to reduce or augment it, and spatially distinct portions exert these contrasting influences.