THE NERVOUS SYSTEM 527 



spinal tract (not distinguishable) is emerging from its nucleus of origin, the 

 nucleus ruber. Just below this level its fibres decussate (ventral decussation of 

 Forel) and pass to the location they have been seen to occupy in preceding levels. 

 The nucleus ruber or red nucleus is very conspicuous, occupying a large part of 

 the reticular formation. It consists of a large-celled part which gives rise to the 

 rubro-spinal tract and to rubro-bulbar fibres, and a smaller-celled portion. 

 The latter sends fibres to the thalamus and thence to the pallium. The nucleus 

 ruber, probably the first named part, also receives fibres from the pallium. 

 According to this it will be seen that the nucleus ruber is a part of the descend- 

 ing cerebello-rubro-bulbar and spinal path (XVII, p. 484), of the cerebello- 

 pallial path (VII, p. 483), and of the pallio-rubro-bulbar and spinal path (XIV, 

 p. 483). The red nucleus probably also receives collaterals from the colliculo- 

 spinal tract. 



The medial longitudinal fasciculus is diminished, some of its descending 

 fibres having been formed from reticular formation nuclei below this level. 

 Many of its fibres, both ascending and descending, send collaterals and terminals 

 to the cells of the oculomotor nucleus. 



Efferent Suprasegmental Neurones. — The ventral part of the brain is com- 

 posed of a mass of efferent pallial fibres, the pes pedunculi. The pyramidal fibres 

 from the precentral areas of the cerebral hemisphere (pallio-spinal and some 

 pallio-bulbar fibres) occupy about the middle three-fifths, but are also scattered 

 through other parts of the pes, especially the mesial part. The leg fibres are 

 probably more numerous laterally, the arm fibres in the middle, and the face fibres 

 mesially. In the lateral part of the pes are pallio-pontile fibres from the occipital 

 and temporal lobes of the cerebral hemispheres (occipito-temporal pallio-pontile 

 fibres). In the mesial part are efferent pallial fibres from the frontal lobe, in part 

 to the pons nuclei (frontal pallio-pontile) and in part possibly from the lower 

 frontal region to the motor nuclei of cranial nerves VII and XII. Besides the 

 above pallio-pontile and pallio-spinal fibres in the pes there are two other 

 aberrant peduncular bundles (p. 523) which probably contain efferent pallial 

 fibres which pass to the motor nuclei of nerves V, VII and XII. These two 

 bundles, which may be termed the mesial and lateral pedunculo-tegmental 

 bundles ("medial accessory fillet" and "lateral peduncular fillet") detach them- 

 selves from the pes higher up, and below this level come to lie in the vicinity 

 of the medial lemniscus. 



Dorsal to the pes and constituting the remainder of the basis pedunculi is a 

 mass of gray matter which, on account of the pigmentation of its cells, is known 

 as the substantia nigra. The substantia nigra receives collaterals from the ad- 

 joining pes fibres. These are probably fibres from the motor cortex but according 

 to some may be from the corpus striatum or corpus subthalamicum. The 

 axones of the cells of the substantia nigra enter the tegmentum. Their des- 

 tination is unknown. 



The superior cerebellar peduncle has completed its decussation below this 

 level and its fibers are seen surrounding or within the nucleus ruber which is one 

 of their terminal nuclei. Other fibres of the superior cerebellar peduncle 

 terminate in the thalamus and some are stated to terminate in the nucleus 

 of nerve III. 



