THE PONS AND THE CEREBELLUM. 



131 



not, like those of the oculo-motor, pass backward through the 

 tegmentum. They run directly caudad from their point of 

 origin, then turn upward and decussate with the corresponding 

 fibres of the opposite side in the velum medullare anticum. 

 Thus they emerge from the brain on its dorsal aspect, just 

 behind the corpora quadrigemina. Portions of the course of the 

 trochlear fibres are shown in Figs. 76 and 77. According to 

 J. Stilling, the nerve also receives a rootlet from the cerebellum. 

 The whole course of the nerve, as obtained from dissections, is 

 shown in Fig. 67. Oculo-motor fibres are no longer given off 



KftwH. 



FIG. 76. FIG. 77. 



(From a newborn child. Haematoxylin stain.) 



FIG. 76. Section through the ending of the corpora quadrigemina. The decussation 

 of the cerebellar peduncles almost completed, the red nucleus very small. Fibres of the 

 trochlear nerve to be seen above. 



FIG. 77. Section through the velum medullare anticum, in which can be seen the 

 trochlear nerve. The red nucleus has disappeared, the cerebellar peduncles lie almost 

 at the periphery. The substantia reticularis occupies the situation formerly held by the 

 red nucleus. In the pons is a little fasciculus of medullary fibres. All the rest of the 

 fibres of the crusta are non-medullary, and are only shown in outline. 



at this level. The posterior longitudinal fasciculus and the 

 stratum lemnisci, however, are continued down to this level. 

 They still occupy the same relative positions which they did in 

 the quadrigeminal region, as may be seen by comparing Figs. 

 72, 75, 76, and 77. In sections through the commencement of 

 the pons the substantia nigra has disappeared from view. Thus 

 it comes that the layer of the fillet is no longer separated from 

 the pes, but lies directly upon it. 



If we have once formed a clear idea of the different struc- 

 tures to be seen in a good section through the quadrigeminal 

 region, we shall have no difficulty in recognizing and locating 



