THE MESENCEPHALON. 
opposite side of the same geniculate body the mesial root 
takes shape on the surface, and the appearance is such 
_ very naturally be arrived at that the inferior brachium 
_ tract are continuous with each other under the geniculate 
| elevation. This is not the case, however; the fibres of 
the inferior brachium, to a large extent, proceed into 
491 
of origin of the optic tract 
that the conclusion might 
and this root of the optic 
PULVINAR 
/ CORP:GEN:EXT: 
LATERAL ROOT 
OPTIC TRACT 
vA | 
the subjacent tegmentum under cover of the internal 
~ geniculate body and help to constitute an ascending 
~ tract from the inferior quadrigeminal body, which pro- 
ceeds upwards to the optic thalamus. Of the fibres 
of the mesial root of the optic tract some end in the SS ERLAMeREE Seiya 
gray matter of the internal geniculate body, whilst St 
_ others arise within it. They constitute what is called 
Gudden’s commissure. ane: 
The superior brachium (brachium quadrigeminum 
superius) is carried upwards and forwards between the overhanging pulvinar and 
the corpus geniculatum internum. A surface examination of the mesencephalon i is 
sufficient to show that, while a large part of this strand enters the corpus 
geniculatui externum, a considera! le portion runs into the lateral root of the 
optic tract. 
The optic tract is thus attached to the brain-stem by two roots, viz. a mesial 
- and a lateral, which are separated from each other by a distinct groove. The 
mesial root disappears under the internal geniculate body. The lateral root spreads 
out and some of its fibres enter the superior quadrigeminal body through its 
brachium, whilst others find their way into the corpus geniculatum externum and 
— the pulvinar of the optic thalamus. 
| Crura cerebri (pedunculi cerebri)—The crura cerebri constitute the chief bulk 
of the mesencephalon (Fig. 364, p. 492). Upon the basal aspect of the brain they 
appear as two large rope-like strands, which emerge close together from the upper 
aspect of the pons Varolii and diverge as they proceed upwards to enter the 
cerebrum. At the place where each crus disappears into the corresponding side 
— of the cerebrum, it is encircled by the optic tract. 
Each crus cerebri is composed of two parts, viz. a dorsal tegmental part (teg- 
mentum), which is prolonged upwards into the region below the optic thalamus 
(subthalamic tegmental region), and a ventral pedal. portion or crusta (basis pedun- 
euli), which, when traced upwards into the cerebrum, is seen to take up a position 
on the outer side of the optic thalamus and to be continuous with the internal 
capsule. When the base of the brain is examined it is the crusta which is seen, 
and it is observed to be white in colour and streaked in the longitudinal direction. 
In the tegmentum the longitudinally- arranged fibres are, for the most part, cortici- 
petal, or, in other words), fibres which are ascending towards the cortex of the 
cerebrum; the crusta, on the other hand, is entirely composed of longitudinal 
strands of fibres which are corticifugal or fibres which descend from the cortex 
cerebri. 
On the surface of the mesencephalon the separation between the tegmental and 
pedal portions of the crus cerebri is clearly indicated by an inner and an outer groove. 
The inner or mesial furrow is the more distinct of the two. It looks into the 
interpeduncular space, and from it emerge the fascicles of the third or oculo-motor 
nerve. It is, therefore, termed the sulcus oculo-motorii (sulcus nervi oculo-motoriu). 
The outer groove, which is placed on the lateral aspect of the mesencephalon, 
receives the name of the sulcus lateralis mesencephali. When traced downwards, it 
is observed to become continuous with the furrow which intervenes between the 
middle and superior peduncles of the cerebellum. 
A close inspection of the outer surface of the tegmental part of the crus cerebri, 
below the level of the quadrigeminal brachia, will reveal some faintly-marked 
bundles of fibres curving obliquely upwards and backwards to reach the inferior 
quadrigeminal body (Fig. 335, p. 449). These are fibres of the lateral fillet, coming 
to the surface at the sulcus lateralis and sweeping over the subjacent superior 
cerebellar peduncle to gain the inferior quadrigeminal body. 
~ MESIAL ROOT. 
\. SCORP:GEN:INT: 
“ SUPR. BRACHIUM. 
363.—DIAGRAM OF THE Roots 
OF THE Optic NERVE. 
| 
