THE CENTRAL NERVOUS SYSTEM. 351 
B. MESENCEPHALON.—3. Mesencephalon.—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). Its roof forms 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 ¢). 
The cranial pair (g) (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, y; Fig. 
141, 2), a portion of the roof of the ’tween-brain. The caudal 
corpora (Fig. 141, ) 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 (#2) 
stretches between their caudal borders. The brachium of the 
caudal corpus quadrigeminum (brachium quadrigeminum in- 
ferius) (7) extends craniad and disappears beneath a consider- 
able elevation, the corpus geniculatum mediale (s). 
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, 0). 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 
