498 THE NERVOUS SYSTEM. 
Here many of its fibres end in terminal arborisations around the thalamic cells; 
whilst others, according to certain observers, emerge from the thalamus, enter the 
posterior limb of the internal capsule, and ascend through the corona radiata to the 
Rolandic area of the cerebral cortex, 
in which they, for the most part, end 
in the posterior central convolution 
(Flechsig). 
Ganglion Interpedunculare and Fasci- 
culus Retroflexus.—On the ventral aspect 
of the tegmentum close to the surface and to 
the mesial plane, there is a small group of 
cells in the lower part of the gray matter 
which forms the locus perforatus posticus. 
This is termed the ganglion interpedunculare. 
The fasciculus retroflecus is a small band 
of fibres which arises above in the ganglion 
habenulee—a nuclear mass which will be 
studied in connexion with the diencephalon 
—and which runs downwards and forwards 
in the tegmentum of the upper part of the 
mesencephalon between the inner aspect of 
the nucleus ruber and the mesial plane, to 
end in the ganglion interpedunculare. 
The Fountain Decussation.—If the 
region in front of the posterior longitudinal 
bundles be examined in the upper part of 
the mesencephalon a very close decussation of 
fibres in the mesial plane will be observed 
in the interval between the two red nuclei. 
This is the “ fountain decussation.” Accord- 
ing to Held, the fibres which take part in the 
dorsal portion of the fountain decussation (de- 
cussation of Meynert) come from the superior 
quadrigeminal bodies, and, after they have 
gained the opposite side, they turn down- 
wards in the posterior longitudinal fasciculus. 
The ventral part of the decussation (decussa- 
Fic. 370.—DIAGRAM OF THE CONNEXIONS OF THE |. = : , 
Mestal FicLer AND. ALSO OF CERTAIN or tan Won of Forel) would appear. to betormedsing 
T'HALAMO-CORTICAL FIBRES } arcuate fibres of the tegmentum which arise 
in the gray matter of the Sylvian aqueduct. 
Crusta or Pes of the Crus Cerebri (basis pedunculi).—The crusta presents a 
somewhat crescentic outline when seen in section, and it stands quite apart from 
its fellow of the opposite side. It is composed of a compact mass of longitudinally 
directed fibres, all of which, as Déjerine has shown, arise in the cortex of the 
cerebrum and pursue an unbroken corticifugal course into and through the crusta 
of the crus cerebri. These fibres may be classified into two distinct sets, viz. 
cortico-pontine and pyramidal. 
The cortico-pontine fibres possess this leading peculiarity: in their course down- 
wards they are all arrested in the ventral part of the pons Varolii and end in 
fine terminal arborisations around the cells of the nucleus pontis. They come from 
certain well-defined areas of cerebral cortex, viz. (1) the cortex of the prefrontal 
part of the frontal lobe, and (2) the cortex of the middle portion of the temporal 
lobe. These tracts would appear to hold a very definite position within the crus. 
Thus it has been satisfactorily established that the temporo-pontine strand forms the 
outer or lateral fifth of the crusta, whilst the recent researches of Ferrier and 
Turner render it more than likely that the fronto-pontine strand holds a similar 
position in the inner or mesial part of the crusta. 
The pyramidal fibres constitute the great motor tract from the cerebral cortex. 
They occupy a position corresponding to the middle third of the crusta. The pyra- 
midal tract differs from the cortico-pontine strands in being carried downwards 
through the ventral part of the pons and on the ventral aspect of the medulla into 
the cord, which it enters in the form of the crossed and direct pyramidal tracts. 
On its way through the pons and medulla it sends fibres to the various motor 
nuclei in those sections of the brain-stem, 
