trigeminus. From the sensory trigeminal fibres entering the pons, collateral branches are given off, which pass as reflex collaterals to the nucleus of the N. facialis. The efferent path lies within the ocular facial.
In place of the reflex collaterals, however, individual neurones may transfer the impulse from the sensory to the motor tracts. For example, a sensory fibre on entering the spinal cord may transfer the impulse first to cells, whose axis-cylinders do not leave the cord, as do those of the motor anterior horn-cells passing to the periphery, but enter the white substance and divide into ascending and descending branches. These division-branches, after a longer or shorter course, end within the gray substance of cord-segments of higher or lower levels (Fig. 123, b). First within these segments occurs the transference to true motor cells. In this manner, not only the motor cells of the same level are impressed, but the stimulus is carried to higher and lower lying cord-segments and, consequently, transferred to a larger number of motor neurones.
The further possibility exists, that the impulse may be conducted from the spinal cord by the ascending tracts to the higher lying subcortical centres and that first here the transference to the motor paths occurs. The resulting movements are mostly more complicated than the simple reflexes, although, as in the case of the latter, they are unconsciously executed. As shown in Fig. x23, the impulse may be conducted through
certain paths from the spinal cord to the cerebellum, thence to the nucleus ruber, and from the latter be carried downward to the cord and, finally, to the muscle.
The second intercentral connection between the sensory and motor parts is fur nished by association conduction. By means of the latter, a conscious voluntary action is rendered possible, by means of the system of association fibres within the cerebral hemispheres. An impulse is carried through the sensory path as far as the cerebral cortex and here transferred to cells within a certain sense-centre ; in these cells, it may be assumed, the material stimulus is released, which corresponds to psychic sensation. Obe may simply imagine, that from this locality the impulse is transferred by an additional neurone directly to the cells in the motor cortical region and thence farther carried by the motor tract. The cortical connection is, however, far more complex, since only after traversing numerous intermediate neurones does the impulse finally reach the motor centre and from there pass to the motor path, since cooperation of the various cortical centres must be assumed in explanation of the complex psychic processes.