143 



The aviisncs is for the orii^ia Df the cerebsllu-n, in the 

 phylogeny of the vertebr-ites, as a fusei outgrowth of the pair 

 of tuberoala acustica. The organ represents a specializatioT 

 of that part of the oblongata forming the original terminal 

 station for the acustico-lateral systeii. 



The Viibrairi appears to have its organization arrange! 

 c:)ntribatory to the roof-nacleis. This is a group of giant 

 neurones, the axones of which enter into and very largely compose 

 the fibre of Reissner. The roof-nucleus and Reissner's fibre, 

 together with the motor neurones of the spinal cord, provide a 

 direct path for aotor reflexes between certain senses and the 

 body lusculature. The senses thus nediated are, primarily, the 

 olfactory and the visual, but the acustico-lateral and the gen- 

 oral cutaneous systems may be represented also. 



The stratum medullars profundum is an important highway 

 into which there are traceable optic fibres, axones from the 

 tectum mesencephali, fibres of the olfactory apparatus, and 

 fibres fr^m posterior regions. Fibres emerge from the stratum 

 to terminate near the cells of the roof-nucleus. 



The tectum mesencephali receives practically all of the 

 optic fibres in Mustelus. Three zones of neurones are recog- 

 nizable in the tectum, with optic terminations in all of them. 

 The deepest of the three layers has a more generalized struct- 

 ure than the corresponding zone of the teleost. 



