INTERNAL STEUCTURE OF THE PONS. 



569 



.creased in size by the addition of the fibres of the spino-thalamic tract. Further, 

 ; t e floor of the fourth ventricle becomes narrower, and other objects appear in the 

 gmental substance. 



The brachium conjunct! vum (O.T. superior cerebellar peduncle) is a very con- 



licuous object in sections through the middle and upper parts of the pons. In 



ansverse section it presents a semilunar outline, and as it emerges from the 



.rebellum it lies immediately on the lateral side of the fourth ventricle, towards 



hich its concave aspect is turned (Fig. 500). Its dorsal border is joined with the 



irresponding brachium of the opposite side by the thin lamina of white matter, 



: rmed the anterior medullary velum, whilst its ventral border is sunk to a small 



tent in the dorsal part of the pons. As it is traced upwards it sinks deeper and 



eper into the pons until it becomes completely submerged, with the exception 



the dorsal border to which the anterior medullary velum is attached. It now 



is on the lateral side of the tegmental or reticular substance of the pons, and this 



sition it maintains until the mesencephalon is reached (Fig. 501). 



About half -way up the pons the nuclei of the trigeminal nerve mark a very 



Upper end of fourth ventricle 

 Mesencephalic root of the 

 trigeminal nervi 



Medial longitudinal bundle 



natio reticularis 

 iinnisci lateralis 



Trigeminal nerve 



Anterior medullary velum 



Gray matter on floor of fourth 

 ventricle 



Brachium conjunctivum 



Lemniscus lateralis 



Commencing decussa- 

 tion of brachia 

 conjunctiva 



Lemniscus medialis 



Transverse fibres 

 of pons 



Pyramidal 

 bundles 



'in. 501. -SECTION THRODQH THE SUPERIOR PART OP THE PONS OF THE ORANG, ABOVE THE LEVEL 



OF THE TRIGEMINAL NUCLEI. 



iportant stage in its tegmental portion. These nuclei are two in number on 



t h side, viz., a large oval terminal nucleus for certain of the sensory fibres of 



i nerve and a nucleus of origin, equally conspicuous, for certain of the motor 



f res (Fig. 500). The sensory nucleus lies close to the lateral surface of the pons, 



1 ply sunk in its tegmental part, and in the interval between the submerged 



a ,erior border of the brachium conjunctivum and the ventral part of the pons. 



I e motor nucleus is placed on the medial side of the sensory nucleus, but somewhat 



,'^rer the dorsal surface of the pons. At this level the tractus spinalis of the 



^eminal nerve begins by the bending downwards of the fibres of the sensory 



The sensory and motor roots of the fifth nerve traverse the ventral part 



the pons on their way to and from the region of the nuclei. 



ove the level of the nuclei of the trigeminal nerve a new tract of fibres 



into view. This is the mesencephalic root of the trigeminal nerve, as it 



s towards the rest of the nerve. It is a small bundle of nerve-fibres, 



mar in cross section, which lies close to the medial side of the brachium 



unctivum and on the lateral and deep aspect of the gray matter on the floor 



3 fourth ventricle (Figs. 500 and 501). 



i a slightly deeper plane than the mesencephalic root of the fifth nerve, 

 >,ween it and the medial longitudinal bundle, and in close relation to the gray 



