SIR CHARLES BELL ON THE BRAIN. 477 



an anterior one for motion, a posterior one for sen.sation, and a middle one, which for 

 the present we may call the tract of the corpus olivare. 



After these dissections, it is impossible for us to consider the medulla oblongata as 

 the mere commencement of the spinal marrow : it has a peculiar structure and di- 

 stinct functions ; it is the body formed by the convergence of the great tracts of the 

 cerebrum, where these tracts respectively meet and decussate ; in it the tract of the 

 corpus olivare is joined to those of motion and sensation. 



Below the medulla oblongata the spinal marrow commences, or rather is prolonged 

 from it, but it is constituted with a distinct arrangement of its columns. On each 

 side it receives three columns from the cerebrum, besides those which come down 

 from the cerebellum, under the name of corpora restiformia, to form its posterior 

 part, and these columns enter into relations which do not exist above. 



Decussation of the Posterior or Sensitive Part. 



We have noticed a fact of more than ordinary importance as reconciling the occur- 

 rence of symptoms, with our knowledge of anatomy. Where the posterior tract, de- 

 scending from the cerebrum, has reached the point of the medulla oblongata, just op- 

 posite to the decussation of the corpora pyramidalia on the fore part, we described a 

 coalescence. We have already stated, that when we proceed to separate the columns 

 on the sides of the slit called calamus scriptorius, we see small, neat, and regular fila- 

 ments, as it were, interlacing and joining the two columns. But when we examine 

 further, we perceive that these filaments belong to a plate of fibrous texture which 

 passes in the central plane from before backwards (Plate XX. fig. b.). This striated 

 septum stops or is interrupted by the union of the columns of sensation ; and now 

 attending to the fibres of these two columns, we find them to decussate with an inter- 

 weaving as distinct as that of the corpora pyramidalia or anterior columns (Plate XX. 

 fig. 2. c). After this union and decussation has taken place we may trace the nervous 

 matter downwards in the two lateral portions of the spinal marrow, covered by the 

 columns, which are the most posterior of all, and which descend from the cerebellum 

 under the name of corpora restiformia. 



Before tracing the origin of the sensitive roots of the spinal nerves, and that of the 

 fifth nerve, in their relations to these tracts, we may review their course. We cannot 

 fail to observe the remarkable correspondence in the structure and course of the two 

 grand tracts or divisions of the crus cerebri, which descending, form so large a portion 

 of the spinal marrow. Tracing them from the brain, we find both converging from 

 the periphery of the hemisphere ; both entering masses of cineritious matter, emerging 

 alike, and approaching, but not absolutely joining ; both contracting into narrow 

 pyramidal columns ; both having corresponding decussations, and only distinguish- 

 able at last by one of them giving origin to the motor nerves, and the other to the 

 sensitive. 



MDCCCXXXIV. 3 Q 



