666 



STRUCTURE OF THE BULB, PONS, AND MID-BRAIN [CH. XLVI. 



fourth, sixth, and twelfth nerves, which are close to the middle line ; 

 and (2) the motor nucleus of the fifth, the nucleus of the seventh, and 

 the nucleus ambiguus (motor nucleus of the ninth and tenth nerves) 

 which form a line more lateral in position. 



It should be added that, except a portion of the optic nerve fibres, 

 a few fibres of the third, and the whole of the fourth nerves, none 

 of the fibres of the cranial nerves cross to the opposite side. 



The first two pairs of cranial nerves, the olfactory and the optic, 

 will be studied in connection with smell and vision later on. 



SUP. PED. OF CEREBELLUM 

 MIDDLE ., 



CEREBELLAR 



4 

 HEMISPHERE 



FIG. 409. Diagrammatic representation of dorsal aspect of medulla, pons, and mid-brain. 



We can now pass to the consideration of transverse sections of 

 this part of the central nervous system. We will limit ourselves to 

 seven, the level of which is indicated in the above diagram (fig. 409). 

 The cerebellum has been bisected into two halves and turned out- 

 wards, its upper peduncles having been cut through to render the 

 parts more evident. The position of our seven sections is indicated 

 by the transverse lines numbered 1 to 7. 



First section (fig. 410). This is taken at the lowest level of the 

 bulb, through the region of the decussation of the pyramids. The 

 similarity to the cervical cord will be at once recognised; the 



