THE NERVOUS SYSTEM 523 



(distinguishable in one-month infant, where they are medullated while the pallio- 

 pontile system is not). Bundles apparently forming lateral and mesial portions 

 of the medial lemniscus (not indicated in the figure) are aberrant efTerent i)allial 

 fibres. Such bundles have been seen passing from pons to tegmentum and 

 also imbedded in the medial lemniscus in lower levels (pp. 507, 497). Some of 

 these fibres are possibly fibres acting directly or indirectly on the efferent periph- 

 eral neurones or motor nuclei of the cranial nerves (see p. 527). 



The pallio-pontile fibres are still more numerous. The gray matter in the 

 pons (nuclei pontis) is very extensive. 'J'he transverse fibres of the pons no 

 longer pass at this level into the cerebellum, but are collected at the sides of the 

 pons to pass backward to the cerebellum (compare with an external view of the 

 brain). 



The superior cerebellar peduncles or brachia conjunctiva are two large 

 crcscentric bundles of fibres in the lateral part of the reticular formation. 

 Some of their fibres have begun to decussate in the ventral part of the reticular 

 formation. 



The colliculo-spinal tract or predorsal fasciculus lies ventral lo the medial 

 longitudinal fasciculus. 



Midbrain or Mesencephalon 



The dorsal surface of the midbrain presents four rounded promi- 

 nences, the two inferior and two superior colliculi (posterior and 

 anterior corpora quadrigemina) . Ventrally are seen two diverging 

 masses of longitudinal fibres, the pes pedunculi, separated by a deep 

 groove or sulcus. In the midbrain are to be distinguished, (a) the 

 expanded roof, the colliculi or corpora quadrigemina, (b) the tegmentum 

 containing the segmental (cranial nerves IV and III) and interseg- 

 mental apparatus and the afferent suprasegmental paths, and {c) the 

 basis pedunculi, ventral to the tegmentum and comprising the 

 principal efferent pallial paths {pes pedunculi) and the substantia 

 nigra. 



The cavity of the midbrain is the aquediictus Sylvii or iter. 



PRACTICAL STUDY 



10. Transverse Section through Midbrain at Level of Superior Colliculi ("An- 

 terior Corpora Quadrigemina) and Exit of Nerve III (Oculomotorj (Figs. 



332 and 355) 



Compared with the preceding section, the following arc the most conspicuous 

 changes: The roof has now enlarged into the superior colliculi; the tegmentum 

 now contains the nuclei and roots of nerve III and the red nucleus; instead 

 of the pons, the ventral part of the brain is now composed of the basis pedunculi, 

 consisting of a mass of efferent pallial fibres and the substantia nigra. The 



