CONDUCTION PATHS OF THE MESENCEPHALON.
The mesencephalon, which, as the smallest of the brain segments, includes the quadrigeminal region and the cerebral peduncles, is traversed by several main tracts, on the one hand, and is the termination or the origin of many fibre-strands, on the other. I. The chief tracts traversing the are those which descend from the cortex of the cerebral hemispheres, already described in connection with the tion paths of the telencephalon, namely : the frontal pontile tract, the pontile tract and the motor tract. These three chief paths pass through the basis culi or crusta, the frontal fibres occupying the medial and the occipito-temporal the lateral part, while the motor tract appropriates the middle portion of the crusta between the pontile tracts. An additional traversing path is the sensory tract or medial fillet, which ascends from the spinal cord, the nuclei of the posterior columns and the end-nuclei of the sensory cerebral nerves and continues to the thalamus, joined by the tegmental tract that unites the thalamus with the sensory 'region in the cortex of the parietal lobe. This ascending sensory tract, however, does not pass through the crusta of the cerebral pe duncle, but through the tegmental area. Of the additional traversing paths, the tractus thalamo-olivaris or the central tegmental tract deserves special mention.
II. Tracts ending within the mid-brain : a. Within the region of the superior colliculus end some fibres of the optic tract ; within the inferior colliculus, the fibres or collaterals of the lateral fillet, which latter represents the primary auditory path.
b. Within the quadrigeminal region end additional fibres from the cortex, tradus the superior colliculus principally fibres from the occipital lobe and in the inferior colliculus those from the temporal lobe ; further fibres are those of the tractus which ascend from the lateral column of the spinal cord.
c. Within the nucleus ruber end, first, fibres from the cortex (frontal lobe, regio opercu laris) and from the corpus striatum; second and most important, fibres from the cerebellum. The last take their origin in the nucleus dentatus, in small part also in the nucleus tecti, and, perhaps, also in the cortex of the cerebellum, and form the brachia conjunctiva or superior cerebellar peduncles. After decussation within the tegmentum of the mid-brain, the fibres end in the nucleus ruber and, in part, also in the d. Small individual which in part end in the mid-brain and in part run still farther caudally—the tegmental bundle of the ganglion interpedunculare, fibre strands from the corpus mamillare and from the posterior longitudinal bundle.
III. Tracts arising within the mid-brain: a. Tractus et which arise from the deep medullary substance of the corpora quadrigemina, cross (Meynert's tegmental decussation) and terminate within the nuclei of the medulla oblongata and within the anterior and lateral columns of the spinal cord. Since fibres of the visual tract end within the superior colliculus and those of the auditory tract within the inferior colliculus, the impulses brought to the mid-brain by these paths may be conveyed to the medulla and the cord by the tecto-bulbar and the tecto-spinal tract respectively. These paths, there fore, are also called the reflex tract. The tract leading to the anterior column is known also as the fasciculus longitudinalis praedorsalis, since the bundle lies ventral to the posterior longitudinal bundle in its course through the brain-stem.
b. Tractus from the quadrigeminal plate to the cerebellum.
c. Tractus (Milnzer), a small fibre-strand that arises within the quadrigeminal region, more especially within the inferior colliculus, and ends in the pontile nuclei in the vicinity of the pyramidal tract.
A small bundle, the tractus of Pavlow, extends from the quadrigem inal region to the tegmentum of the pons and ends within the nucleus reticularis tegmenti.
d. Tractus also known as Monakow' s bundle, arises in the red nucleus of the tegmentum. The fibres emerging from the nucleus cross and descend through the pontile tegmentum and the medulla oblongata to the lateral column of the spinal cord, to end within the anterior horn.
Moreover, the tractus includes fibres which pass, crossed and un crossed, from the red nucleus to the formatio reticularis of the pons and of the medulla oblongata ; a further bundle, the tractus passes from the red nucleus to the nucleus of the lateral fillet.
e. Fascicularis longitudinalis medialis, commonly called the posterior longitudinal bundle, is composed of fibres taking their origin in different places. The principal fibre strands arise from Deiters' nucleus and from the nucleus of the posterior commissure and of the posterior longitudinal bundle, situated in front of the oculomotor nucleus (p. 182).
f. Finally it must be noted, that the oculomotor and the trochlear nerves, as well as a small motor root of the trigeminus, have their origin within the mid-brain.