THE CENTRAL NERVOUS SYSTEM. 351 



B. MESENCEPHALON. 3. Mcsencephalon. The mesen- 

 cephalon or midbrain includes the corpora quadrigemina (Fig. 

 141) and the pedunculi cerebri (Fig. 142). In the primary 

 midbrain there is a pronounced thickening of the walls accom- 

 panied by a reduction of the central canal. The midbrain does 

 not thus become very large and is concealed in the dorsal view 

 by the cerebellum and cerebrum, though its floor appears in 

 the ventral view just craniad of the pons (Fig. 138, g). Its 

 narrow canal is the aqueductus cerebri (aqueduct of Sylvius) 

 (Fig. 143,7). I ts ro f f rms the corpora quadrigemina, and 

 its floor the pedunculi cerebri. 



In a dorsal view (Fig. 141) the roof is seen to be marked 

 by two pairs of elevations, the corpora quadrigemina (/ and q}. 

 The cranial pair (q) (known as the colliculi superiores) are cir- 

 cular in outline, surrounded on all sides except the cranial one 

 by a deep groove. From the cranial side a tract of fibres 

 (brachium quadrigeminum superius, or arm of the cranial cor- 

 pus) extends craniad and disappears beneath the thalamus (/). 

 Between the anterior or cranial corpora quadrigemina lies the 

 pineal body or epiphysis (corpus pineale) (Fig. 143,^/5 Fig. 

 141, 2), a portion of the roof of the 'tween-brain. The caudal 

 corpora (Fig. 141, p] are larger than the cranial ones, and 

 ovoid in shape with the long axis vertical. They are united 

 in the median line, and the velum medullare anterius (;) 

 stretches between their caudal borders. The brachium of the 

 caudal corpus quadrigeminum (brachium quadrigeminum in- 

 ferius) (r) extends craniad and disappears beneath a consider- 

 able elevation, the corpus geniculatum mediale (j). 



Crossing this brachium is seen a small tract of fibres which 

 extends ventrad, crosses the pedunculus cerebri, and reaches 

 the medial border of the latter. It is the tractus transversus 

 peduncularis (Fig. 142, b\ Ventrad of the caudal corpus 

 quadrigeminum is seen a triangular area of oblique fibres which 

 corresponds in position to the human lemniscus. 



The pedunculi cerebri (peduncles of the cerebrum) form 

 the ventral part of the midbrain. They appear in a ventral 

 view of the entire brain (Fig. 138) as two broad tracts of fibres 

 (g) emerging from beneath the pons and diverging from one 



