494 THE NERVOUS SYSTEM. 
reticularis of the medulla and the dorsal or tegmental portion of the pons into the 
mesencephalon. It therefore consists of fine “bundles of longitudinal fibres inter- — 
sected by arching fibres, which take a transverse and curved course. The interstices — 
between these nerve-bundles is occupied by gray matter containing irregularly 
scattered nerve-cells. On its dorsal aspect the tegmentuin is continuous, at the 
side of the Sylvian gray 
matter, with the bases of 
Mesencephalie root of fifth the corpora quadrigemina, 
7 Deve whilst ventrally it is separ- 
__Nueleus of fourth nerve ated from the crusta by 
the substantia nigra. The 
two tegmenta of opposite 
sides are, to some extent, 
marked off from each other 
in the mesial plane by a 
prolongation upwards of 
the median raphe of the 
pons and medulla, al- 
though, in the lower part 
of the mesencephalon, this 
is much obscured by the 
decussation of the superior 
peduncles of the cerebel- 
lum. Thetwolongitudinal 
strands, termed the pos- 
terior longitudinal bundle 
and the fillet, are pro- 
od longed upwards through- 
out the entire length of 
the mesencephalon ; and 
they present the same 
ST A en ee ne es relations tothe tegmentum 
Fic, 365.—TRANSVERSE SECTION THROUGH THE HUMAN MESENCEPHALON : 2 
AT THE LEVEL OF THE INFERIOR QUADRIGEMINAL Bopy. as in the lower parts of 
the brain. The former is 
placed in relation to its dorsal aspect, whilst the fillet is carried up in its ventral 
part. 
The tegmentum of the crus cerebri may be considered as presenting two parts : 
viz. (1) a lower part, which is placed subjacent to the inferior quadrigeminal bodies 
and which is largely occupied by the decussation of the superior cerebellar peduncles 
(Fig. 365); and (2) a superior part, subjacent to the superior quadrigeminal 
bodies, which is traversed by the emerging bundles of the third nerve and which 
contains a large and striking nuclear mass, “termed the nucleus ruber or the red teg- 
mental nucleus (Fig. 566). In the lower part of the mesencephalon is the nucleus 
of the fourth nerve; in the upper part, the nucleus of the third nerve. 
Superior Cerebellar Peduncles (brachia conjunctiva)—As the superior cere- 
bellar peduncles leave the pons and sink into the tegmenta of the mesencephalon, 
they undergo a complete decussation subjacent to the alae quadrigeminal 
bodies and the Sylvian gray matter (Figs. 347, p. 467; 367, p. 496; and 365). 
In this manner each peduncle is transferred from one side across the mesial — 
plane, to the opposite side. The decussation is completed at the level of 
the upper borders of the inferior quadrigeminal bodies, and then each peduncle — 
proceeds upwards into the superior part of the tegmentum, where it encounters — 
the red nucleus. Into this a large proportion of its fibres plunge and come toan | 
end in connexion with the nuclear cells. Many of the peduncular fibres, how- | 
| 
| 
ee = - Inferior quadrigeminal body 
Sylvian gray 
matter 
Aqueduct of 
sylvi achi inferius 
Sylvius Brachium inferi 
Raphe 
Superior 
cerebellar 
peduncle 
Substantia__— 
nigra 
ever, are carried around the nucleus so as to form for it a capsule which is thicker 
on the inner than on the outer side (Fig. 566). These are prolonged into the 
subthalamic region, and ultimately reach the ventral aspect of the optic thala- 
mus, into which they proceed. A certain number of these fibres end in connexion — 
with the cells of the thalamus, whilst it is held by certain observers that the 
