566 



THE NEKVOUS SYSTEM. 



trigemf 



present the form of two compact bundles. Superiorly, however, they are broken up 

 into smaller bundles by the transverse fibres of the pons, and are spread out over a 

 wider area. At the upper border of the pons they again come together and form two 

 solid strands, each of which is continuous with the central part of the correspond- 

 ing basis of the cerebral peduncle. Added to these there are twice as many other 

 fibres entering the pons from the basis pedunculi to terminate in the nuclei pontis 



The fibrae pontis at the inferior border of the pons are placed on the superficial 

 or ventral aspect of the pyramidal bundles. As we proceed upwards they increase 

 in number, and many are seen breaking through the pyramids and even passing 

 across upon their dorsal aspect. Laterally, these transverse fibres are collected togethe: 

 into one compact mass, which enters the white central core of the cerebellum anc 

 constitutes the brachium pontis (O.T. middle cerebellar peduncle). At the mediai 

 plane the transverse fibres of the two sides of the basilar portion of the pon 

 intercross and form a coarse decussation. 



The nuclei pontis form a considerable part of the bulk of the basilar portio] 

 of the pons. The gray matter is packed into the intervals between the intersectm 

 transverse and longitudinal bundles. 



There is some analogy between the pyramidal portions of the medulla oblongata an 

 the ventral part of the pons. In the medulla oblongata fine arcuate fibres, on their way 1 

 the surface, pass through the pyramids. Other external arcuate fibres sweep over tl 

 surface of the pyramids. These present a strong resemblance to the transverse fibres i 



the pons. They likewise rea( 

 the cerebellum, although I 

 a different route, viz., tl 

 restiform body. The nucl 

 pontis are represented also 

 the pyramidal part of t 

 medulla oblongata by t 

 arcuate nuclei, which a 

 covered over by the exterr 

 arcuate fibres, and even te: 

 to penetrate, to a slight < 

 tent, into the pyramidal trac 



VRhomt)ic lip> These arcuate nuclei, as alrea 



pointed out, are continue 

 with the nuclei pontis. 



Connexions of t 

 Longitudinal and Trai 

 verse Fibres. When 



FIG. 499. DIAGRAM OF THE LEFT LATERAL ASPECT OF THE FOSTAL trans verse section through 1 

 RHOMBENCEPHALON REPRESENTING SOME OF THE CELL GROUPS superior part of the pons 

 AND FIBRE TRACTS. compared with one close to 



inferior border, it becomes 



once apparent that the numerous scattered bundles of longitudinal fibres which en ' 

 the ventral part of the pons from above, if brought together into one tract, would fc i 

 a strand very much larger than the two pyramids which leave its lower aspect and en ' 

 the medulla oblongata. It is clear, therefore, that many of the longitudinal fib 

 which pass into the pons from above do not pass out from it below into the medi i 

 oblongata. What becomes of these fibres that are thus absorbed in the pons? 1 

 known that the pyramidal bundles suffer a small loss by the fibres which they send to - 

 nuclei of origin of the efferent nerves which arise within the pons (viz., the motor roo f 

 the trigeminal, abducens, and facial nerve nuclei) ; but this loss is, comparatively speak 

 trifling. It is clear, therefore, that other longitudinal bundles enter the pons from at e 

 apart from those which form the pyramidal tracts. These bundles occupy a lateral J 

 dorsal position in the ventral part of the pons, and may be termed the cerebro-pom e 

 fibres, seeing that they come from the cerebral cortex and end in fine ramifications aro d 

 the cells of the nuclei pontis (Fig. 498). 



The transverse fibres take origin as axons of the cells of the nuclei pontis. Cros g 

 the median plane, they enter the brachium pontis of the opposite side, and thus r< h 

 the cerebellar cortex, where they end in ramifications round certain of the cortical c s. 

 Some authorities believe that there are also fibres passing in the opposite direction. * 



Fasciculus 



circumolivaris 



pyramidis. 



,8rchium 

 ponfis. 



CEREBELLAR 

 RUDIMENT. 



Flocculus. 



Recessus laferalis 

 vcnrriculi quarti. 



MEDULLA OBLONCATA. 



nl-o -bulbsre. 



