NERVOUS CENTRES. (Human Anatomy. Tur Encrpnaton.) 
pons Varolii ; and on its superior surface, which 
looks backwards, are the corpora quadrigemina, 
the processus cerebelli ad testes, and part of 
the floor of the fourth ventricle (fig. 386). 
According to Chaussier, its weight is equal 
to about the sixtieth or sixty-fifth part of the 
entire brain. 
~ We shall describe separately the inferior and 
the superior surfaces of this segment of the en- 
cephalon, and its intimate structure as unfolded 
by sections. } 
it The inferior surface, (pons Varolii, annular 
_ protuberance, ) convex from side to side, is inter- 
rupted along the median plane from behind for- 
wards by a shallow groove in which the basilar 
artery usually lies, giving off in its course nu- 
merous minute capillaries to the nervous struc- 
ture of the mesocephale. 
___ When the pia mater has been stripped off 
_ this surface, it is seen to be very evidently 
composed of a series of transverse fibres which 
take an arched course. The fibres are collected 
into large fascicles separated from each other 
by very distinct intervals, so that there is no 
_ part where the fibrous structure is more appa- 
rent than here. They form ares of circles, not 
" concentric, lying one behind the other in a se- 
fies nearly parallel. Owing to this want of 
_ complete parallelism the width of this surface 
_ measured from before backwards is much less 
at each extremity than in the centre. The an- 
terior margin is convex, and forms a thick edge 
_ crossing the crura cerebri like a bridge; hence 
the term pons was applied by Varolius to the 
"whole series of fibres. The posterior border is 
concave, less curved than the anterior, and 
crosses the anterior pyramids and olivary co- 
lumns, as the latter does the crura cerebri. The 
‘intervening fascicles of fibres become gradually 
less curved as they approach the posterior 
margin. 
These transverse fibres form a stratum of 
considerable thickness at the inferior surface of 
the mesocephale. Some grey matter is depo- 
sited between the less superficial layers which 
constitute it. The more deep-seated layers are 
penetrated and crossed at right angles by the 
ascending fibres of the anterior pyramids. A 
remarkable interlacement takes place at this 
Situation between the vertical and transverse 
fibres—the latter passing alternately in front of 
and behind adjacent bundles of the former. 
Some of the vertical fibres seem to sink into 
and connect themselves with the grey matter. 
A transverse vertical section of the meso- 
cephale gives a more complete view of the 
exact extent of the transverse fibres. They are 
found to occupy rather more than one-third of 
the depth of the exposed surface. Their dispo- 
sition in lamine is very apparent. Those which 
are nearest the centre of the mesocephale have 
between them considerable intervals, which 
are filled up by grey matter, through which 
pass Rariically the fibres of the pyramids. The 
imtervals between the laminz gradually dimi- 
nish towards the inferior surface of the pons, 
and the quantity of intervening grey matter 
mes proportionally less, and disappears 
altogether from between those lamine the in- 
685 
tervals of which are not traversed by the fibres 
of the pyramids. 
The transverse fibres pass on either side into 
each hemisphere of the cerebellum, contributing 
with the processus cerebelli ad testes and the 
restiform bodies to form the crura cerebelli. 
They are the inferior peduncles of these crura. 
The anatomy of these transverse fibres evi- 
dently denotes that they serve to connect the 
right and left cerebellar hemispheres, as com- 
missures, and in a manner strikingly analogous 
to that in which the fibres of the corpus callosum 
connect the cerebral hemispheres. This view 
of the office of these fibres is strongly confirmed 
by the fact that their number is always in the 
direct ratio of the size of the lateral hemispheres, 
and that when the hemispheres are absent, these 
fibres no longer exist. When, therefore, the 
cerebellum consists only of a median lobe, there 
is no pons Varolii. 
Some of the transverse fibres nearer the in- 
ferior surface appear to dip in along the me- 
dian line, and to pass upwards and backwards, 
forming a vertical plane of fibres which divides 
the mesocephale into two symmetrical portions, 
and Chaussier imagined that a decussation 
took place at this situation. The groove in: 
which the basilar artery lies is formed partly 
by the greater condensation which is produced 
along the median plane by this arrangement, 
and partly by the slight bulging on either side 
of it, caused by the ascent of the anterior py- 
ramids. These fibres are continuous with a 
series of similar ones in the medulla oblongata 
(antero-posterior fibres of Cruveilhier). 
The extent of the superior surface of the me- 
socephale may be limited in front by a line 
which passes from side to side just before the 
anterior of the corpora quadrigemina, and pos- 
teriorly by the base of the valve of Vieussens. 
This occupies a much greater space than the 
inferior surface. It is an inclined plane, and 
passes downwards and backwards, being con- 
cealed by the anterior lamine of the superior 
vermiform process of the cerebellum and the 
posterior border of the corpus callosum. 
The corpora or tubercula quadrigemina are 
four rounded eminences—gangliform bodies— ~ 
disposed in pairs (fig. 386, D, D’). The ante- 
rior pair are larger than the posterior. The for- 
mer have been distinguished. as the nates, the 
latter the ¢estes.* These bodies are situate further 
forwards than the pons, and are chiefly connected 
with the superior surface of each crus cerebri. 
The nates are of a deeper grey colour than 
the testes. In this respect they resemble the 
optic thalami. Both pairs are similar in struc- 
ture to those bodies. When cut into, they ap- 
pear to consist of fibrous matter intermingled 
with vesicular. Thin sections examined with 
the microscope exhibit intricate interlacements 
of tubular fibres with vesicular matter inter- 
posed—a true ganglionic structure. 
An important fact deserves special notice as 
indicating that vesicular matter is found in 
* In reference to these absurd appellations Willis 
has the following remark : ‘ Prominentia orbicularis 
—quarum usus longé nobilior videtur, quam ut 
viliora ista natium et testium nomina mereantur.” 
