NERVOUS CENTRES. (!!UMAN ANATOMY. THE ENCEPHALON.) 



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 

 I have 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 

 system as those which form the pons Varolii. 

 They are largely developed in some quadru- 

 peds, although theyassumeadifferentform. The 

 fibres which constitute what Treviranus called 

 the trapezium, appear to answer the same 

 purpose 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- 

 racterizes them in the human subject. These 

 fibres cross the anterior surface of the medulla 

 oblongata parallel lo 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, 

 and also with the hemispheres of the brain 

 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 die 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 exceptions, 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 

 predict that the lesion on which it depends will 

 be 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 

 Aretoeus,* 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.f 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 



* Tllfi anicuv xai s-nfj-timv ^oviav ffaScav, Q$\. A, 

 x<f>. , p. 87, Ed. Kuhn. 



t 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 protuberantias cre- 

 pidine atque maxiine in imo medullaris caudicis 

 qua in spinalem abit- In priore itaque annularis 

 protuberantias pane, qua superius reflexa pro com- 

 prehendendis oblongata? medulla? cruribus in an- 

 guli formam interius producta tenuatur, sic ex 

 concurrentibus fibris, strictiorique agmine coeun- 

 tibus altera alteram scandit ut praeter mirum im- 

 plexum decussatio lucnlentissime appareat. Idip- 

 sum ferine in postica ipsius crepidine occurrit. Eo 

 iterum in loco, qui quarto ventriculo subjicitur, 

 pneter varios fibrarum ordines et colores, in adver- 

 sum latus productas et decussatas fibras commode 

 spectavimus. Si ea tamen evidenter uspiain con- 

 spicitur, profecto qua in evidentissime duas vix 

 lineas infra pyramidalia atque adeo olivaria cor- 

 pora conspici potest. Qua euim in longitudinem. 

 producta linea seu rimula pyramidalia corpora dis- 

 cernuntur, si leniter deducantur, probe prius eo 

 potissimum loco arctissime h;erente tenui meninge 

 nudata, non tenues decussari fibrillas, sed validos 

 earundem fasciculos in adversa contcnderc, quam 

 aportissime demonstrabunt." Ohorv. Anat. cap. iii. 

 $ xii. p. 61. Ed. Lugd. Bat. 1739. 



