THE MESENCEPHALON 835 



mid-line as a bridge over the opening of the cerebral aqueduct into the third ventricle. This is 

 the posterior commissure of the cerebrum, and contains commissural fibres arising in both the 

 thalamencephalon and mesencephalon. The triangular area bounded by the stem of the epi- 

 physis, the thalamus, and the superior colliculus with its brachium, is known as the habenular 

 trigone. 



Inferiorly, the lamina quadrigemina is continuous with the isthmus of the 

 rhombencephalon by way of the brachia conjunctiva or superior cerebellar pedun- 

 cles, and the anterior medullary velum which bridges between the mesial margins 

 of these peduncles. The narrowed upper end of the velum, the part directly below 

 the inferior quadrigeminate bodies, is thickened into a well-defined white band 

 known as the frenulum veil. 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 mesen- 

 cephalon, a lateral depression between the base of this ridge and the cerebral 

 peduncle below and continuous into the transverse sulcus at the superior border 



Fig. 659. — Diagram of Lateral View of Mesencephalon and Adjacent Structures. 



(After Gegenbaur, modified.) 



Pulvinar of thalamus 



Epiphysis 



Lateral geniculate body Z^-^^T ^^^^ ^^'^"^ geniculate body 



Quadrigeminate bodies 

 Cerebral peduncle. 



- Lateral lemniscus 



Po'*^ C^-~«^^>)'^^ ^"^ ^J'^^5__Supenor cerebellar peduncle 



-Middle cerebellar peduncle 

 Inferior cerebellar peduncle 



Olive 



of the pons. The ridge is thickened laterally by the lateral lemniscus, which is 

 disposed as a band of white substance passing obliquely upward from under the 

 brachium pontis, applied to the lateral surface of the brachium conjunctivum and 

 which enters the lateral margin of the mesencephalon. The region at which the 

 lateral lemniscus approaches nearest the surface and in which the largest portion 

 of its nucleus lies is the slightly elevated trigone of the lemniscus. 



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 somewhat, producing 

 the interpeduncular fossa, and in so doing pass upward and lateralward to disap- 

 pear beneath the optic tracts (fig. 629). The posterior recess of the interpeduncu- 

 lar fossa extends slightly under the superior margin of the pons, while its anterior 

 recess is occupied by the corpora mammillaria of the prosencephalon. The tri- 

 angular floor of the fossa is the posterior perforated substance, a greyish area 

 presenting numerous openings for the passage of blood-vessels. It is divided bj' 

 a shallow median groove and is marked off from the medial surface of each peduncle 

 by the oculomotor sulcus, out of which emerge the roots of the oculomotor nerves. 

 The ventral surface of each peduncle is rounded and has a somewhat twisted 

 appearance, indicating that its fibres curve from above medialward and downward. 

 Sometimes two small, more or less transverse bands of fibres may be noted crossing 

 the peduncle — an inferior, the taenia pontis, and a superior, the transverse pedun- 



