THE PONS VAROLII. 



61 



The nucleus of the facial nerve (n. VII} lies in the reticular formation just 

 dorsal to the superior olivary nucleus, and at some depth, therefore, below the floor 

 of the fourth ventricle. It begins to be visible in sections immediately above the 

 medulla oblongata, in the form of small isolated groups of nerve-cells, from which 

 separate bundles of fibres proceed, and extends three or four millimeters upwards. Its 

 cells are of moderate size, and their axis-cylinder processes are directed inwards and 

 backwards towards the grey matter of the floor of the ventricle, where they collect 



P" 



VI 



Fig. 50. SECTION ACROSS THE LOWER, PAKT OF THE PONS (after Stilling and Schwalbe). 



Py> pyramid-bundles continued up from the medulla ; po, transverse fibres of the pons passing from 

 the middle crus of the cerebellum, before (po 2 ) and behind (po') the chief pyramid bundles ; t, deeper 

 transverse fibres, constituting the trapezium ; the grey matter between the transverse fibres is not 

 represented either in this or in the following figures; r, raphe ; o.s., superior olivary nucleus; a. V, 

 bundles of the ascending root of the fifth nerve, enclosed by a prolongation of the grey substance of 

 Rolando ; VI, the sixth nerve ; n. VI. its nucleus ; VII, the facial nerve ; Vila, ascending portion of 

 the facial root ; n. VII, its nucleus ; VIII, superior root of the auditory nerve ; n. VIII, part of the 

 nucleus of Deiters ; v, section of a vein. 



to form a longitudinal bundle, oval in section, which runs for a short distance 

 upwards in the grey matter and then turns sharply in a ventrolateral direction, 

 traversing the thickness of the pons to emerge on its lateral aspect (fig- 41, B.) 

 From facts brought forward by Mendel and supported by Tooth and Turner, the 

 facial nerves appear to receive some fibres through the posterior longitudinal bundles 

 from the oculomotor nuclei ; these fibres probably being those for the frontalis, 

 corrugator supercilii, and orbicularis palpebrarum. On the other hand the fibres to 

 the orbicularis oris may arise from the hypoglossal nucleus. 



The motor nucleus of the fifth nerve (fig. 51, nV) comes to view 

 in higher sections through the pons, situated a little below the surface close to 

 the lateral margin of the fourth ventricle. It contains large pigmented multipolar 

 nerve-cells, the axis-cylinders of which pass out into the motor root of the trigeininal. 

 Fibres are also seen in the grey matter of the floor of the ventricle connecting both 

 this motor nucleus (and also the sensory nucleus, immediately to be described) with 

 the raphe, and through this probably with higher parts of the brain. 



The so-called upper sensory nucleus of the fifth nerve (n. V) lies on the 



