The efferent impulses of the cerebro-pontal tracts are evi dently not voluntary motor; they probably regulate and coor dinate the functions of the lower motor neurones.
Paths Through the Red Nucleus (Fig. 148).—Impulses run from the opercular part of the cerebral cortex in the frontal lobe to the red nucleus. From the red nucleus they pursue a direct route to motor nuclei, cranial and spinal.
The Rubro-spinal Path.—By this route, impulses run through the crossed descending tract of the red nucleus to the cerebral nuclei and gray crescent in the opposite side of the spinal cord. Crossing the median raphe at once, in the hypothalamic region through the ventral tegmental decussation (Foreli), the im pulses descend by way of the rubro-spinal tract, through the ven tral part of the formatio reticularis of the mid-brain and pons, in the medial part of the lateral fillet, then through the lateral column of the medulla, among the fibers of the ventral spino cerebellar tract, and finally down the spinal cord, through the field ventral to the lateral pyramidal tract, to their destina tion in the gray matter. From the gray crescent they proceed, with or without transferring, to the root neurones of the spinal nerves and are conducted to the muscles.
The red nucleus is also a very important station in the coor dinating reflex arc concerned with locomotion (Horsley).
Paths Through the Thalamus.—The thalamus is a center of consciousness for the impulses of pain and temperature (Head and Holmes) and it receives impulses from many parts of the cere bral cortex. Thalamic impulses descend through the thalamo spinal tract to the motor nuclei of the brain-stem and the gray crescent of the cord. This tract accompanies the rubro-spinal tract; but whether or not it decussates with the rubro-spinal tract is at present undetermined. Through the thalamus and red nucleus two very important strio-fugal paths reach the motor nuclei and convey to them steadying influences. Again, the thalamus is an important' station in several reflex systems.
Reticulo-spinal Paths.—The reticulo-spinal tract is the name suggested for the tracts originating in the nuclei of the reticular formation, chiefly in the pons, and descending to the gray matter of the spinal cord. There are two of them on either side. The anterior reticulo-spinal tract accompanies the medial longitudinal bundle down the anterior ground bundle of the cord without decussating, unless the crossing occurs near the termination. The lateral reticulo-spinal tract which is crossed decussates in the brain stem near its origins and descends in the lateral column of the spinal cord. Just how the impulses reach the nuclei centrales and nuclei laterales of the formatio reticu laris, in which the reticulo-spinal tracts take their origins, cannot be definitely stated; but, having arrived in them, they descend to both crescents of the spinal cord and apparently enter into all its segments. The anterior nerve roots complete the paths.
Short Fiber Paths.—Those are paths in the formatio reticu laris chiefly. (I) Impulses having reached the great ganglia of the cerebrum and mid-brain may run through many relays down the formatio reticularis of mid-brain, pons and medulla and the antero-lateral fasciculus proprius of the spinal cord, to the gray crescent of the same, and continue through the anterior root fibers to their destination. On the other hand, the impulses, leaving formatio reticularis in mid-brain, pons or medulla, may enter the nuclei of motor cerebral nerves and be conducted by them to the muscles and glands supplied by cerebral nerves. (2) The impulses may leave the formatio reticularis in the pons, and run to the cerebellar cortex through the brachium pontis. From the cerebellum they may follow the ordinary course through the cerebello-spinal path, by way of the nucleus of Deiters, to the anterior gray columna of the cord. (3) Impulses having arrived at the thalamus, may descend the thalamo olivary tract to the inferior olivary nucleus of the medulla and pass to the cerebellar cortex of the opposite side, through the olivo-cerebellar fibers. The tlialamo-olivary fasciculus runs through the middle of the tegmentum of mid-brain and pons and the lateral column of the medulla to the dorso-lateral wall of the inferior olive; the olivo-cerebellar fibers cross over in the medulla to the opposite restiform body, through which the im pulses reach the cerebellar cortex. Any impulses in the cortex of the cerebellum may pass through the cortico-nuclear fibers to the cerebellar nuclei and the cerebello-tegmental fibers from these nuclei, via brachium conjunctivum and restiform body, directly to motor cranial nerve nuclei; and, through the rubro spinal, thalamo-spinal and the vestibulo-spinal tracts, they may continue to the motor nuclei of cranial and spinal nerves. (4) Through certain short fibers in the medial longitudinal bundle which rise in the nucleus of the sixth cerebral nerve and cross to the opposite nucleus of the motor oculi, impulses run from the nucleus of the abducent through the opposite third nerve to the internal rectus oculi. They explain the conjugate action of the eyes in health, and also the conjugate deviation observed in lesions affecting the nucleus of the sixth nerve. In nuclear lesions of the abducent nerve the external rectus of the same eye and the internal rectus of the other eye are para lyzed if the nucleus is destroyed and stimulated if the nucleus is only irritated.