THE STRUCTURE OF THE BRAIN STEM 371 



the upper part of the pons, the division into the forma tio reticularis or 

 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 



Inf. corpus quadrigeminum 



Grey matter^- ' ,-' ^Mesenc. root of 5th n. 



.Nucleus of 4th nerve 

 Aqueduct of Sylvius^^ gg^ ^^ ^^ 



-Postr. long, bundle 



Mesial fillet 



Substantia nigra. 



Crust a 



FIG. 187. Transverse section through human mid-brain, on level of the inferior 

 corpora quadrigemina. (CUNNINGHAM.) 



tegmental portion we may distinguish on each tfide 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 cerebellum. 



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 

 to say that it consists of a middle and two lateral lobes. The surface of the 



