THE NERVOUS SYSTEM 505 



still further increased in size. External arcuate fibres may be present and prob- 

 ably contain fibres to the restiform body and possibly fibres from the cerebellum, 

 which end in the reticular formation. The arcuate nuclei are present. The 

 central tegmental tract is larger. 



Intersegmental Neurones. — The reticular formation is very extended. Its 

 composition (longitudinal and transverse fibres, and cells) should be examined 

 carefully. The rubro-spinal tract is in the same position, but the lateral Deitero- 

 spinal tract is now more internally located. Its fibres cannot usually be dis- 

 tinguished, but are bending inward and toward its nucleus of origin (lo- 

 cated somewhat higher). The medial longitudinal fasciculus may be partially 

 separated from the medial lemniscus. It is a complex bundle and contains at va- 

 rious levels (c) descending and ascending fibres from Deiters' nucleus and other 

 cells scattered in the reticular formation, (/)) descending fibres from the nucleus 

 of the medial longitudinal fasciculus in the tegmentum of the m.idbrain. The 

 fibres of this fasciculus probably terminate in many nuclei, especially those of 

 eye-muscle nerves (III, IV and VI) (comp. Figs. 339, 345 and 358). 



Efferent Suprasegmental Neurones. — The pyramids and colliculo-spinal 

 tracts are in the same positions. The aberrant efferent pallial fibres already 

 noted (p. 497) may be seen in the lemniscus. 



6. Section through the Hindbrain at Level of Junction of Pons and Cerebellum 

 and Entrance of Vestibular Nerve (Figs. 332 and 340) 



The most conspicuous features are the nuclei and fibres of the pons, added 

 ventrally to the preceding structures, which are now collectively known as the 

 tegmentum; the cerebellum, enclosing dorsally the fourth ventricle; and the connec- 

 tions {inferior and middle peduncles, arms, or brachia) of the cerebellum with the 

 rest of the brain. The greater part (restiform body) of the inferior peduncle 

 represents ascending cerebellar connections from all parts below this level, the 

 middle peduncle (from the pons) is the second link in the descending connection 

 from the pallium to the cerebellum (pallio-cerebellar path). The superior ped- 

 uncle (efferent cerebellar) is not fully formed at this level. (Comp. p. 514.) 



Efferent Peripheral Nevirones. — In this level and that of the next section are 

 present the nuclei and root fibres of nerves VII and VI. 



The nucleus of nerve VII, or nucleus facialis is seen occupying a lateral posi- 

 tion in the reticular formation similar to that of the nucleus ambiguus. In it 

 may be made out the usual plexus of fine terminals and the coarser root fibres 

 which proceed dorso-mesially to the floor of the ventricle, where they partially 

 envelop the nucleus of the VI (usually not present in this level). They then 

 turn cephalad, forming a compact longitudinal bundle (next section) and finally 

 turn ventro-laterally and caudally to emerge on the lateral aspect at the caudal 

 border of the pons. This latter part is the second part as distinguished from 

 the first part of the connection. The bend is known as the genu facialis. Ac- 

 cording to some authorities, some of the fibres cross and pass out in the root of 

 the opposite side. 



Four groups of cells composing the nucleus facialis have been distinguished: 

 three ventral groups which, passing from the most mesial to the most lateral 

 group, innervate respectively the muscles of tympanum, of pinna and of mouth 



