CHAPTEK XLYI 



STRUCTURE OF THE BULB, PONS, AND MID-BRAIN 



WE may study the bulb and pons by examining first the anterior 

 or ventral, then the posterior or dorsal aspect, and last of all the 

 interior. 



Anterior Aspect. 



The bulb is seen to be shaped, like an inverted truncated 

 cone, larger than the spinal cord, and enlarging as it goes up until 

 it terminates in the still larger pons (fig. 406, p). In the middle line 

 is a groove, which is a continuation upwards of the anterior median 

 fissure of the spinal cord; the columns of the bulb are, speaking 

 roughly, continuations upwards of those of the cord, but there is a 

 considerable rearrangement of the fibres in each. Thus the prominent 

 columns in the middle line, called the pyramids (a a), are composed 

 of the pyramidal fibres, which in the spinal cord are situated princi- 

 pally in the lateral columns of the opposite side (crossed pyramidal 

 tracts). The decussation or crossing of the pyramids (b) occurs at 

 their lower part: a small collection of the pyramidal fibres is, 

 however, continued down the cord in the anterior column of the same 

 side of the cord (direct pyramidal tract): these cross at different 

 levels lower down in the cord. 



On the outer side of each pyramid is an oval prominence (c c), 

 which is not represented in the spinal cord at all. These are called 

 the olivary bodies or olives ; they 'consist of white matter outside, 

 with grey and white matter in their interior. 



The restiform bodies at the sides (d d) are the continuation upwards 

 of those fibres from cord and bulb which enter the cerebellum, and 

 the upper part of each restiform body is called the inferior peduncle 

 of the cerebellum. 



Posterior Aspect. 



Fig. 407 shows a surface view of the back of the bulb, pons, and 

 mid-brain. Again we recognise some of the parts of the spinal cord 



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