NERVOUS CENTRES. (Human Anatomy. Tue Encepsaton.) 
of the pons Varolii are the least curved, and 
in some rare instances the uppermost ones ex- 
hibit no more curvature than the posterior fibres 
of the pons. 
Both Santorini and Rolando have figured 
_ these fibres in their most highly developed 
state. The delineation given by the latter au- 
thor, whilst it serves admirably as a diagram to 
show the general relation of the fibres, repre- 
sents them as more numerous and distinct than 
_ Thave ever had an opportunity of seeing them, 
, and likewise exhibits them as passing upwards 
through the pons. This is certainly not the 
case. These fibres appear to incorporate them- 
selves with the restiform bodies which connect 
the medulla oblongata with the cerebellum. 
It seems to me that the arciform fibres may 
be properly regarded as a part of the same 
stem as those which form the pons Varolii. 
They are largely developed in some quadru- 
os although they assumeadifferent form. The 
bres which constitute what Treviranus called 
| the trapezium, appear to answer the same 
pu as the arciform fibres; but, by reason 
_ of the non-developement of the olivary bodies 
on the exterior of the medulla oblongata, they 
do not take the curved course, which cha- 
 yacterizes them in the human subject. These 
fibres cross the anterior surface of the medulla 
oblongata parallel to but distinct from the pons 
_ Varolii. They connect the pyramids and res- 
_ tiform bodies on each side. 
_ By their continuation upwards the pyramidal 
___ bodies form a connection with the mesocephale, 
e and also with the hemispheres of the brain 
y through the medium principally of the corpora 
_ striata, and perhaps also of the optic thalami. 
_ Through the decussation of fibres which takes 
place just before the pyramids sink intothe spinal 
cord, each cerebral hemisphere is connected 
with that half of the spinal cord which belongs 
to the opposite side of the body. By this ar- 
rangement is explained the influence which 
cerebral disease exercises upon the side of the 
body opposite to that on which it occurs. If 
the right hemisphere be irritated, convulsions 
are produced on the left side; if the right he- 
misphere be compressed, the left arm and leg 
and side of the face will be paralysed. So 
constant is this “ crossed” influence of cerebral 
lesion that it can be attributed only to some 
uniform physical condition of the nervous cen- 
tres. And that the anatomical disposition on 
which it depends is situate at the lower part 
of the medulla oblongata is proved, not only 
by the existence of these decussating fibres at 
this situation, but likewise by facts revealed 
by the phenomena of disease, and the results 
of experiment. Morbid lesions, for example, 
which have their seat above the decussation are, 
with rare execptions, accompanied by affection 
of the opposite half of the body—those which 
involve the nervous centre below the decussation 
affect the body on the same side. Mechanical 
injury to the brain or spinal cord produces like 
effects. And so constantly is this the case that 
when we meet a case of paralysis or of con- 
vulsion affecting only one side, we confidently 
ges that the lesion on which it depends will 
e found on the opposite side of the brain. 
681 
This law of cerebral action has been known 
from the earliest periods of medical science, 
but the anatomical explanation of it, the sugges- 
tion of which dates as far back as the time of 
Areteus,* has been generally admitted only 
within a comparatively recent period. This 
explanation was founded on the hypothesis of 
a decussation of fibres in the medulla oblon- 
gata to a greater or less extent. Santorini, in- 
deed, laid it down that decussation took place 
not only in the lower part of the medulla ob- 
longata, but likewise at the anterior and pos- 
terior margins of the pons Varolii.+ But it is 
quite impossible, by our ordinary means of ob- 
servation, to detect any such connection be- 
tween the anterior pyramids elsewhere than at 
their inferior extremity. In many instances I 
have thought that the fibres of the commissure 
which forms the floor of the anterior fissure 
presented an appearance as if decussation took 
place along the entire length of the pyramids. 
But the numerous foramina by which the com- 
missure is penetrated to give passage to vessels 
for the central substance of the medulla, are 
very apt to give rise to a fallacious appearance 
of this kind. 
It has been stated that there are exceptions 
to this law of cerebral action. Such certainly 
must be extremely rare, for in the course of a 
considerable experience for many years I have 
not met with an unequivocal instance in which 
paralysis occurred on the same side with cere- 
bral lesion. The analysis which Burdach has 
given of 268 cases of paralysis in which there 
was lesion of a single hemisphere, shows very 
strikingly how rare must such an exception be. 
Of these cases he states that 10 were accom- 
panied with paralysis of both sides, and that 
258 had hemiplegia. And of the hemiplegic 
cases, the paralysis occurred on the same side as 
the cerebral lesion in only 15. 
The full explanation of these exceptions has 
* Tlep: asrimy nar onetiov ypovsav maw, BBA. Ay 
xep. ¢, p. 87, Ed. Kuhn. 
+ Santorini must have been well acquainted with 
the decussating fibres of the pyramids, which he 
clearly describes. The whole passage is worth 
being quoted here. ‘ Id autem triplici potissimum 
in loco animadvertere potuimus ; in utraque scilicet 
priore, posterioreque annularis protuberantiz cre- 
pidine atque maxime in imo medullaris caudicis 
qua in spinalem abit. In priore itaque annularis 
protuberantie parte, qua superius reflexa pro com- 
prehendendis oblongate medulle cruribus in an- 
guli formam interiis producta tenuatur, sic ex 
concurrentibus fibris, strictiorique agmine coeun- 
tibus altera alteram scandit ut preter mirum im- 
plexum decussatio luculentissimé appareat. Idip- 
sum fermé in postica ipsius crepidine occurrit. Eo 
iterum in loco, qui quarto ventriculo subjicitur, 
preter varios fibraram ordines et colores, in adver- 
sum latus productas et decussatas fibras commodé 
spectavimus. Si ea tamen evidenter uspiam con- 
spicitur, profectd quam evidentissimé duas vix 
lineas infra pyramidalia atque adeo olivaria cor- 
pora conspici potest. Qua enim in longitudinem 
producta linea seu rimula pyramidalia corpora dis- 
cernuntur, si leniter deducantur, probé prius eo 
potissimum loco arctissimé herente tenui meninge 
nudata, non tenues decussari fibrillas, sed validos 
earundem fasciculos in adversa contendere, quam 
apertissimé demonstrabunt.” Oberv, Anat. cap. iii. 
§ xii. p. 61. Ed. Lugd. Bat. 1739. 
