THE MEDULLA OBLOXGATA AND TEGMENTUM OF THE PONS. 213 



We are now gradually approaching that region of the pons 

 where the roof of the hind-brain is no longer formed by the 

 cerebellum, but by the velum medullare anticum. The fourth 

 ventricle here begins to narrow down to form the aqueduct of 

 Sylvius. 



The different structures which constitute the tegmentum at 

 this level are very clearly shown in the accompanying non- 

 diagrammatic section through the upper part of the pons of a 

 9 months' foetus. At this period only one small fasciculus of 

 the pes is medullary. In the tegmentum, however, the layer of 

 the fillet, the anterior cerebellar peduncles, the posterior longi- 

 tudinal fasciculus, and many of the fibres of the substantia 

 reticularis are fully developed. The anterior peduncles are 

 continuous above with the velum medullare anticum, upon which 

 rests the anterior extremity of the lingula. Lower down, just 

 above the fillet, can be seen the most posterior of the decussating 

 fibres of the anterior cerebellar peduncles. The descending root 

 of the trigeminus lies, a thin bundle of fibres, on each side of 

 the aqueduct. You must imagine the cells of the locus coeru- 

 leus placed just under the floor of the aqueduct or the rhom- 

 boidal fossa. They did not show well in this specimen. The 

 reticular substance here consists mostly of longitudinal fibres, 

 which cannot be traced higher than the level of the anterior 

 quadrigeminal bodies. Near the middle line on each side is the 

 posterior longitudinal fasciculus. 



From this level up to the quadrigeminal region the appear- 

 ance of the tegmentum does not materially change. The fillet 

 begins to work its way externally and dorsad of the tegmentum 

 to reach the region under the corpora quadrigemina. You 

 remember that these ascending fibres of the fillet could be seen 

 even in sections through the mid-brain. The anterior peduncles 

 approach each other and at last decussate higher up. 



You have already observed the fibres of the crusta and the 

 fasciculi of the pons penetrating them. It should be added that 

 in the anterior levels of the pons there is a system of fibres 



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