452 ANATOMICAL TECHNOLOGY. 



passing laterad and then ventrad just caudad of the postopticus 

 from its ectal origin on the cephalic part of the valvula. 



Observe the contrast in form and position of the opticus and 

 postopticus. The latter is farther from the meson and more sharply 

 defined ; the opticus presents a more gradual slope to the meson. 

 Extending laterad from the lateral slope of the opticus, note the 

 cimbia, the ventral part of which crosses the crus in the Area cru- 

 ralis (Fig. 116, 118 ; PI. II, Fig. 3, and PI. Ill, Fig. 11, cmb.). The 

 cimbia seems to indicate the boundary between the mesencephalic 

 and diencephalic portions of the crus. 



§ 1143. Diencephalon. — Note the great dorso-ventral extent of 

 the diacoelia, the presence of the medicommissura {mcs.) in its dor- 

 sal part, and the absence of any distinct boundary between the 

 diaccElia and the aula. 



The thalami constitute the lateral parietes of the diacoelia, and 

 its ventral and cephalic boundaries are sufficiently obvious, but the 

 dorsal require specification. 



The most caudal part of the roof is formed by the postcommis- 

 sura {pes), which intervenes between the opticus and the cona- 

 rium {en.). 



Between the medicommissure and the fornix the most prominent 

 part, especially with an injected preparation, is a longitudinal 

 plexus, the diaplexus. 



Lift the diaplexus with the tracer at about the middle of its 

 length and note that it is attached throughout by one edge, and 

 that it partly covers a ridge upon the mesal aspect of the thalamus. 

 This, the habena (A.), is widest near its caudal end, which joins its 

 platetrope by a slender band, the Conuuissura habenarum {Os. h.), 

 just dorsad of the conarium. 



The habena becomes narrower and less prominent cephalad and 

 disappears at the cephalic convexity of the thalamus, corresponding 

 with the dorsal limit of the porta. 



Grasp the diaplexus with the fine forceps and gently pull upon it. 

 Note that it readily separates from the other parts excepting at the 

 ends. The ends are connected with somewhat large vessels, but the 

 precise arrangement is not clear to us. Recall the relations of the 

 diaplexus in Menobranchus (§ 1097), where it is free excepting at 

 the cephalic end. 



Remove the diaplexus by carefully cutting the caudal and ce- 

 phalic connections, and note that it was attached to the ventral or 



