EXAMINATION OF THE ENCEPHALIC SEGMENTS. 451 



seen a transverse band (Tractus auditorius) passing from the Emi- 

 uentia auditoria across the wall of the metacoelia. 



1141. EpencepTialon. Tilt the cerebellum caudo-laterad, so 

 as to expose the limits of the epiccelia. Note the distinction be- 

 tween its caudal portion, which extends dorsad into the cerebellum, 

 and the rest, which has for its roof only the thin valvula. Separate 

 the valvula from the cerebellum, noting the continuity of the two at 

 the caudal end of the former. 



With the forceps and tracer tear and push off the lateral part of 

 the cerebellum so as to expose the medipedunculus, the continua- 

 tion of the pons into it. In like manner, remove the caudal part of 

 the cerebellum, and note a less distinct fasciculus, the postpedun- 

 culus, passing from the Tractus lateralis of the metencephalon to the 

 cerebellum just mesad of the medipedunculus. The praepeduncu- 

 lus is exposed by the removal of the cephalic part of the cerebellum ; 

 it forms the lateral wall of the cephalic part of the epiccelia, and is 

 continuous with the base of the postopticus. 



Push the tracer entad of the medipedunculus, and then divide it 

 with the scissors. Grasp the ventral piece and tear off the pons as 

 far as the meson. Note that the pons concealed the continuity of 

 the pyramis with the Cms cerebri. 



1142. MesencepTialon. Note the slenderness of the mesocce- 

 lia, whence the names iter and aquceductus SylmL 



Ventriduct the meten. and epen. so as to leave a space between 

 them and the hemisphere. Remove the caudal end of the latter by 

 a dorso-ventral incision corresponding with the convexity of the 

 splenium. On the cut surfaces, note the darker color of the ectal 

 cinerea, the cortex, as compared with the ental alba. There will 

 be exposed a cavity, the medicornu, and a rounded elevation, the 

 hypocampa. These may be disregarded for the present. 



With the pollex tear away the Lobulus hypocampse (PI. I, 

 Fig. 3, LI. Jimp.} and so much of the hemisphere as readily comes 

 off with it ; then cut off as much more as may be necessary to ex- 

 pose the Tractus opticus (Tr. op.}. 



The parts thus exposed will be found covered by pia, which 

 forms a distinct fold in the F. Tiypocampce (Fig. 121). 



This covering of the mesencephalon and part of the diencephalon 

 is called the velum (interpositum}. Remove it with the forceps, 

 beginning at the postopticus. Note the slender N. trochlearis 



