10? 



profunium to the tarninal arboriz-itions which »e hive ieacri- 

 bad Qsnr the cells of the roof-aacleas. The aeurones of the 

 roof-nucleus transmit such impressions through their axones, 

 Seissner'3 fibre, directly to motor neurones at the several 

 levels of the spinal oori. The apparatjs of the roof-nucleus 

 and Reissner's fibre, regaried as a thing apart, is advantage- 

 ous to V.ustelas because it is a path without relay, a short 

 arc for motor reflexes betvfesn the eye and the olfactory organ, 

 on the one hand, and the body musculature on the other. Tne 

 giant size of the neurones of the roof-nucleus is doubtless the 

 correlative of not only their long axones but of their importance 

 in the econony of the aninal, as /veil. In ascending the scale of 

 the vertebrate series, however, it .vill be found that this mech- 

 anism ever takes on a progressively less and less value, owing 

 to the development of other means for attaining the same end. 



b. The Huoleus of the Oculomotor ius . — The nucleus of 

 the III nerve lies ventral to the aqueduct of Sylvius (Pig. 

 53,n.iri). This collection of neurones is sharply marked off 

 from all surrouniing nervous elements by the large size of its 

 cells and by the greater intensity of stain ^ith lethylen-blue. 

 The group extends antero-posteriorly for some distance. 



The prevailing form of cell is oval, with two or three 

 marked extensions of outline produced by the broaUy triangu- 

 lar bases of the lenirites (Fig. 59). The axone is more slen- 



