70 nULI-F.TIN (.)F THE LABORATORIES 



led with small cells, except alcDg the raphe, where is the usual nucleus, 

 and a short distance on either side, where a disperse nucleus in the 

 position of the fourth still remains. In the lateral jjarts of the area about 

 the acjueduct are a few of the large, nearly spherical or polygonal cells. 

 (" Bladder cells " ol descending tract of the fifth. ) The tracts of the 

 fourth nerve appear as two or three transversely cut bundles at some 

 distance laterad of the aqueduct. The dorsal longitudinal fasciculus 

 lies about one fifth the way from the acpiediict to the ventral surfiice 

 and its lateral portions curve ventrad to almost unite vrith tlie ventral 

 tract. 



Passing cephalad a short distance, the nucleus of the fourth ner\e 

 appears immediately dorsad to the dorsal longitudinal fascicle. It 

 consists here of very irregular ])olygonaI, multipolar cells measuring 

 about .027 mm., u-inle the nucleus of the raphe consists ol a dense 

 aggregate of fusiform elements immediately ventrad to the acjueduct, 

 which at this jjoint is a transverse slit. The dorso-lateral tract now 

 forms a cons])icaous prominence ventrad to the nates fissure. The 

 nates posteriorly exhibit a conspicuous raJi.itin^ structure of the cortex 

 ■which appearance is heightened by the very numerous b'ood vessels 

 which ])ass toward the centre in a direction parallel to that of the nerve 

 chains. Medianly, secUons (VI c-20) display a number of fibre tracts 

 in section, wjiich are doubtless continued from the optic tract and enter 

 oblicpiely from the anterc-median aspect. Proceeding ce})halad these 

 bundles increase in number. 



A comparison of the nates with the optic lobes of lower animals 

 reveals what at first seems a fundamental difference. VvH^iile in rep- 

 tiles and birds the fibres of the 0[)iic tracts pass directly to the periph- 

 ery of the tectum opticum, in mammals these fibres appear to |)ass into 

 the deeper portions. In reptiles pretty definite chains can bj traced 

 from periphery to base of the tectum with insulating, columnar cjn- 

 nective tissue apparatus which forcibly suggests tiie arrangement in the 

 retina. Nothing of this sort distinctly appears in the mammalian nates. 

 The reptilian tectum is characterized by the presence of a few scattered 

 cells of great size which resemble greatly Purkinje's cells of the cere- 

 bellum and are especially abundant in the deeper layers of the interval 

 between the two tecti. Such cells have not been carefully described 

 in mammals, hitherto, to our knowledge, though figured by Bellonci 

 in birds and by Herrick in the alligator, and discovered in great abun- 

 dance ill ihj sam„" portion of the brain of the turtle, by Coppock. In 



