372 



PHYSIOLOGY 



dentate nucleus of grey matter in the cerebellum to the mid-brain, where 

 they decussate below the Sylvian iter and end in the red nucleus and in the 

 thalamus of the opposite side. They also contain the continuation upwards 

 of the antero- lateral ascending tract, which, passing up in the superior 

 peduncles, bends dorsally round the fourth nerve and then, turning back- 

 wards, ends in the superior vermis of the cerebellum. In a section through 

 the upper part of the pons the division into the formatio reticularis or 



4th ventricle 

 Mesenc. root of 5th n. 



Postr. long, bundle 



Form, reticularis 

 Nucleus of lateral fillet 



5tb nerve 



Valve of Vieussens 



Floor of 4th ventricle 



upr. cerebellar 

 peduncle 



Lateral fillet 



Commencing de- 



- cussationof supr. 



cerebellar ped. 



Mesial fillet 



Transverse 

 fibres 



Pyramids 



FIG. 186. Section across upper part of pons Varolii of the orang. (CUNNINGHAM.) 



tegmentum and the part made up of transverse and longitudinal fibres, the 

 pedal portion, is well marked (v. Fig. 186). The fourth ventricle has now 

 become constricted to a narrow canal triangular in section and closed above 

 by the valve of Vieussens. It is surrounded, especially on its ventral side, 

 by grey matter containing the cells of origin of the fourth nerve. In the 

 tegmental portion we may distinguish on each side the superior cerebellar 

 peduncle. Outside the longitudinal fibres of this peduncle are a number 

 of transverse fibres derived from the corpus trapezoides seen in the previous 

 section. To these fibres is given the name of the ' lateral fillet.' They are 

 on their way to end in the roof of the mid-brain in the posterior corpora 

 quadrigemina. The posterior longitudinal bundle lies near the middle line, 

 immediately under the grey matter of the floor of the fourth ventricle, while 

 the longitudinal fibres of the fillet, now called the mesial fillet, form a distinct 

 mass in the ventral portion of the formatio reticularis. The pedal portion 

 contains the longitudinal fibres of the pyramids, now much increased in 

 amount, cut up into bundles by transverse fibres derived from the middle 

 peduncles of the cere'bellum. 



The cerebellum, which covers in the fore part of the fourth ventricle, 

 will have to be described in greater detail later on. At present it will suffice 



