902 



THE NERVE SYSTEM 



The crusta or pes is somewhat crescentic in outline on section and is composed 

 of longitudinal fibre bundles the continuation of the internal capsule divisible 

 into three sectors. The middle sector comprises three-fifths of the cross-section 

 area of the crusta, and comprises the pyramidal tract on its way from the cerebral 

 cortex (motor area) to cranial and spinal centres below. The ectal sector, or 

 lateral one-fifth, comprises the temporopontile tract; 1 its axones arise from the 

 cortical cells in the temporal lobe and end in fine terminal arborizations in relation 

 with cells of the nuclei pontis. The ental sector, or mesal one-fifth of the crusta, 

 comprises the frontopontile tract; its axones arise from cells in the cortex of the 

 frontal lobe and terminate in the nuclei pontis. 



The pyramidal tract is a direct voluntary motor tract; the two corticopontile 

 tracts enumerated above are links in a chain of neurones which constitute an 

 indirect motor tract. The series of neurones in the chain of the indirect motor 

 tract may be shown in the following order: Corticopontile tract; nuclei pontis; 

 cerebellocortex; dentate nucleus; superior cerebellar peduncle; red nucleus; tractus 

 rubrospinalis; spinal gray; spinal nerve; muscle. 

 Summary of the Gray Masses in the Mid-brain: 

 *Central aqueduct gray. 



(a) Oculomotor n. nucleus. 

 (6) Trochlear n. nucleus. 

 Nucleus radicis descendentis nervi trigemini. 

 *Nucleus of medial longitudinal bundle and postcommissure. 

 *Formatio reticularis. 

 *Substantia nigra (intercalatum). 

 *Red nucleus (rubrum). 



*Stratum cinereum of superior corpora quadrigemina. 

 *Nucleus of inferior corpora quadrigemina. 

 *" Interpeduncular" ganglion. 



Structures marked with an asterisk have been considered in the preceding 

 description. The central connections of the oculomotor, trochlear, and trigem- 

 inal nerves may now be described. 



Deep Origin of Cranial Nerves Arising in the Mid-brain. The mesen- 

 cephalic root of the trigeminal nerve has been described on page 886. 



The Trochlear Nerve Nucleus. The trochlear nerve nucleus is situated in the 

 level of the cephalic half of the inferior quadrigeminal body. It is a small 

 oval mass of gray substance in the ventral part of the central aqueduct gray. 

 The cells are large, sometimes stellate in appearance. The root fibres pursue 

 a peculiar course; they accumulate in the latero ventral angle of the aqueduct 

 gray, run caudad, gradually rising dorsad, and suddenly turn mesad to undergo 

 a complete decussation with the root of the opposite side of the superior medul- 

 lary velum, emerging laterad of the frenulum, or at the inner border of the superior 

 peduncle. 



The nucleus is placed under the dominion of the cerebral cortex by pyramidal 

 fibres, and it is associated with other nuclei in the brain stem by the medial longitu- 

 dinal bundle. 



The Oculomotor Nerve Nucleus. The oculomotor nerve nucleus is a group of 

 cell clusters in the ventral portion of the aqueduct gray, subjacent to the superior 

 quadrigeminal body, and extending cephalad to become lost in the gray \vall 

 of the third ventricle at the slope formed by the opening out of the aqueduct. 

 Its nerve elements are arranged in definite groups. The most cephalic of these 

 is composed of smaller elements, closely crowded and embedded in deeply staining 



1 Tiirck's bundle; not to be confused with Tiirck's column in the cord. 



