Humphrey, Brain of the Snapping Turtle. 91 



demarcation. The mesocoele is, in its cephalic portion, a nar- 

 row, dorso-ventral slit widened slightly near its dorsal limit by 

 the caudal continuation of the sulci of Monro. Passing Caudad 

 the cavity rapidly widens extending far laterad into each of the 

 gemina forming the -so-called optic ventricles. Farther caudad 

 the cavity again narrows with its longer diameter dextro-sinis- 

 trad and passes, by the narrow aqueduct, into the metaccele. 

 The colliculi of the mesencephal, first described and figured by 

 Rabl-Ruckhard in the alligator, are shown only slightly devel- 

 oped (Fig. 21). 



Apparently three pairs of nerves originate in the mesen- 

 cephal, but the cephalic pair, optic nerves, mainly terminate 

 here. Originating in the retina the fibers of these nerves grow 

 centrad, reaching the pregemina they expand covering the ce- 

 phalic, lateral and dorsal portions of these lobes. These fibers 

 have no direct cellular connections in the mesencephal but end 

 in arborizations. 



The oculo-motors arise from nidi lying in the cephalic part 

 of the floor of the mesencephal on either side of the ventral 

 slit of the mesoccele. Figure 2 1 shows a transection through 

 the cephalic portion of these nidi. Here the nerve comes off 

 in two main branches. The nidus lies near and ventrad of the 

 main portion of the mesoccele, and dorso-mesad of the posterior 

 longitudinal fasciculus. Farther caudad the nidus extends ven- 

 trad between the posterior longitudinal fasciculus and the ven- 

 tral portion of the mesoccele, reaching nearly to the surface. 

 No decussating fibers were found in this nerve. The trochlearis 

 nerves arise from nidi caudad and slightly dorsad of those of the 

 oculo-motors. These nidi are separated from those of the third 

 nerve by a small space. The fibers arising from the nidi curve 

 latero-caudo-dorsad around the lateral extensions of the meso- 

 coele and meeting in the valvula form a conspicuous decussation 

 (Figs. 22-23.) 



Figure 22 shows a peculiar nidus — the interoptic nidus of 

 Spitzka. This structure was first described by him in a paper 

 upon the brain of Iguana. Subsequently he described the same 

 in the turtles, Nannemys guttata and Chelydra. I have 



