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815 



This is the posterior commissure of the cerebrum, and contains commissural fibres 

 arising in both the thalamencephalou and niesencephalon. The triangular area 

 hounded by the stem of the epiphysis, the thalamus, and the superior colliculus with 

 its brachium, is known as the habenular trigone. 



Inferiorly. the lamina quadrigemina is continuous with the brachia conjunctiva 

 or superior cerebellar peduncles, and with the anterior medullary velum which bridges 

 between the mesial margins of these peduncles. The narrowed upper end of tin; 

 velum, the part directly below the inferior quadrigeminate bodies, is thickened into 

 a well-defined white band known as the frenulum veli. From the lateral margins of 

 this band on each side and just below the inferior quadrigeminate bodies emerge the 

 trochlear nerves (the fourth pair of cranial nerves), and the increased thickness of 

 the band is largely due to the decussation of this pair of nerves taking place within it. 



The brachium conjunctivum, together with the inferior and superior colliculi 

 of each side, form a marked ridge which results in the lateral sulcus of the mesencepha- 



FIG. 607. DORSAL SURFACE OF MESENCEPHALON AND ADJACENT PARTS. (After Spalteholz.) 



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Ion, a lateral depression between the base of this ridge and the cerebral peduncle below 

 and continuous into the transverse sulcus of the superior border of the pons. The 

 ridge is thickened laterally by the lateral lemniscus, which is disposed as a band of 

 white substance passing obliquely upwards from the brachium pontis, applied to the 

 lateral surface of the brachium conjunctivum and which enters the lateral margin 

 of the niesence]ihalon. 



The ventral surface of the mesencephalon is formed by the cerebral peduncles 

 (crura). two large bundles of white substance which are close to one another at the 

 superior margin of the pons, but immediately diverge at a wide angle, producing the 

 interpeduncular fossa, and in so doing pass upwards and lateralwards to disappear 

 beneath the optic tracts (lit:. ">SO). The poxt,riirr recess of the interpeduncular fossa 

 extends slightly under the superior margin of the pons. while its (interior recess is 

 occupied by the corpora mammillaria of the prosencephalon. The triangular floor of 

 the fossa is the posterior perforated substance, a greyish area presenting numer- 



