NERVOUS SYSTEM. (NERVOUS CENTRES. THE ENCEPHALON.) 



684 



space between them, and the central columns 

 do not extend to the surface ; and from the 

 absence of any great developement of grey 

 matter, we find no such arrangement as that 

 which gives rise to the corpus dentutwn in man. 

 The olivary or central columns of the me- 

 dulla oblongata pass into the mesocephale, 

 occupying a plane superior to that of the py- 

 ramidal fibres and of the transverse fibres. 

 They may be traced upwards to the crus cerebri, 

 where they seem to merge into the optic tha- 

 lami, and to form a connection with the corpora 

 quadrigemina posteriorly. 



These columns are seen distinctly in their 

 ascent to the brain in the fourth ventricle, as 

 two cylindrical columns (A, F,Jig. 386). They 

 form the floor of that cavity and are separated 

 from each other by the longitudinal fissure 

 which is continued upwards from the posterior 

 fissure of the medulla oblongata. 



In the fourth ventricle the olivary columns 

 are crossed by the fibres of origin of the 

 portio mollis of the seventh pair of nerves, 

 the white colour of which in the recent speci- 

 men contrasts strikingly with the greyish hue 

 of the columns themselves. We here see dis- 

 tinctly that these columns are the source of 

 origin of these nerves, and no doubt they are 

 equally so of all the nerves which are con- 

 nected with the medulla oblongata, namely, 

 the fifth pair, the eighth, the ninth, and pro- 

 bably also of the sixth. 



The relation of the olivary columns in 

 their upward course, to the other constituents 

 of the mesocephale and crura cerebri, may 

 be conveniently demonstrated in examining 

 tranverse sections of those parts. We shall, 

 therefore, return to this subject in descri- 

 bing the anatomy of those portions of the 

 brain. 



The following interpretation of the various 

 columns of the medulla oblongata, referred to 

 in the preceding description, has much foun- 

 dation in their anatomical relations. 



The olivary or central columns constitute the 

 fundamental part of the medulla oblongata; 

 that, on which its action as a distinct and in- 

 dependent centre depends, and in which the 

 proper nerves of this segment of the ence- 

 phalon are implanted. The continuity of those 

 columns with the optic thalami and corpora 

 quadrigemina materially enhances their phy- 

 siological influence, and denotes their intimate 

 association with some of the most important 

 functions of the brain. And it may be added, 

 that this connection of the medulla oblongata 

 with parts which are ordinarily described as 

 pertaining to the brain itself, shews that the 

 original application of the term by Willis and 

 Vieussens to a much greater extent of the en- 

 cephalon is certainly more consistent with the 

 physiological anatomy than that which is now 

 employed for the convenience of description. 

 There can be no doubt that the extent of this 

 central and fundamental portion of the nervous 

 system is limited above by the optic thalami 

 and below by the spinal cord. 



The anterior pyramids connect the cerebral 

 hemispheres with the spinal cord, the prin- 



cipal bundles of fibres decussating each other 

 on the middle line, so that the right pyramid 

 is the medium of connection by the greater 

 number of its fibres between the right hemi- 

 sphere of the brain and the left half of the cord, 

 but by a much smaller number between that 

 same hemisphere and the right half of the 

 cord. And so also of the left, mutatis mu- 

 tandis. It is highly probable too that the anterior 

 pyramids derive fibres from the locus niger of 

 the crus cerebri and the vesicular matter of the 

 mesocephale. These fibres, therefore, connect 

 those segments with the spinal cord, but whe- 

 ther they contribute to the formation of the 

 decussating or non-decussating bundles, or to 

 that of both, it is impossible to determine. 



The restiform bodies are evidently the con- 

 necting fibres between the hemispheres of the 

 cerebellum and the posterior and antero-lateral 

 columns of the spinal cord. And the pos- 

 terior pyramids connect the posterior part of 

 the medulla oblongata with the cervical and 

 dorsal regions of the cord. 



Nerves. Numerous nerves are connected 

 with the medulla oblongata a fact which 

 serves greatly to enhance its importance as a 

 centre of nervous action. These nerves are the 

 sixth pair, which are connected with the ante- 

 rior pyramids just behind the posterior border 

 of the pons ; the ninth pair, or hypoglossal 

 nerves, which emerge along the anterior border 

 of the olivary body ; the seventh pair (portio 

 mollis and portio dura), which emerge just be- 

 hind the upper extremity of the olivary body ; 

 and the eighth pair, which arise along the pos- 

 terior margin of the olivary body. 



OF THE MESOCEPHALE. The pyramidal 

 and olivary columns may be readily traced, as 

 already explained, from the medulla oblongata 

 up to the cerebral hemispheres ; the former 

 becoming united chiefly with the corpora striata, 

 the latter with the optic thalami. 



In that part of their course which is interme- 

 diate to the medulla oblongata these columns 

 become mingled with certain transverse fibres, 

 and with more or less of vesicular matter, and 

 with them contribute to form a mass which is 

 the connecting link between all the segments of 

 the cerebellum, and may be compared to a rail- 

 road station, at which several lines meet and 

 cross each other. This is the mesocephale or 

 mesenccphule. The name was suggested by 

 Chaussier, inasmuch as it forms " to a certain 

 extent the middle and central part of the ence- 

 phalic organ, the bond which unites the several 

 bundles of fibres which contribute to its for- 

 mation." 



The mesocephale may be isolated from the 

 other segments by dividing the crura cerebri 

 just beyond the anterior margin of the pons, and 

 the crura cerebelli as they penetrate the he- 

 mispheres, and the medulla oblongata on a level 

 with the posterior edge of the pons. The crura 

 cerebri emerge from it in front : the medulla 

 oblongata is connected with its posterior sur- 

 face : on either side it is prolonged into a crus 

 cerebelli. Its inferior surface, which is very 

 convex and looks forwards, is composed of the 

 thick layer of arched fibres which form the 



