302 NORMAL HISTOLOGY. 



the reticulum, other localizations represent important nuclei of cranial 

 nerves. The sheet of gray matter lying in the lower half of the ven- 

 tricular floor is continued over the pons, and there gives rise to 

 nuclei connected with the V, VI, VII, and VIII nerves. While the 

 details of the sections must vary with each plane, the general dis- 

 position of the structures is shown in sections passing through at 

 about the middle of the fourth ventricle. In such sections the dorsal 



FIG. 335. 



Section through upper part of human pons : i, fourth ventricle ; 2, valve of Vieussens lined with 

 ependyma; 2', white matter of anterior medullary velum; 2", gray matter of Hngula ; 3, descending 

 root of trifacial nerve ; 4, substantia ferruginea ; 5, posterior longitudinal bundle ; 6, formatio reticu- 

 laris ; 7, groove indicating boundary between tegmentum and ventral part of pons ; 8, superior cere- 

 bellar peduncle; 9, mesial fillet; 9', lateral fillet; 10, transverse fibres of pons; n, longitudinal 

 fibres ; 12, raphe ; V, trifacial nerve. (After Testut-Stilling.) 



or tegmental portion of the pons bears a resemblance to the me- 

 dulla, the gray dorsal stratum giving rise to fibres which pierce the 

 reticulum in their course to the free surface. 



At a somewhat higher level, lateral groups of pigmented nerve- 

 cells occupy the floor of the fourth ventricle ; these cells are so 

 dark that they collectively present an area visible to the unaided eye, 

 the substantia ferruginea ; seen through the stratum of white 

 fibres forming the immediate floor of the ventricle, this area appears 

 of a bluish-gray or slate-color and constitutes the locus coeru- 

 leus. Close to this pigmented area, lying to its mesial side and near 

 the raphe", an angular tract, known as the posterior longitudinal 

 bundle, extends beneath the gray matter of the ventricle, just at the 

 dorsal border of the reticular formation. This fasciculus, also prom- 



