474 SIR CHARLES BELL ON THE BRAIN. 



the superficial transverse fibres of the pons or nodus cerebri, which passing across 

 terminate in the crura cerebelli. When this part of the pons Varolii is raised, and 

 with it the longitudinal striated matter which passes from the crus cerebri and is 

 prolonged to the corpus pyramidale, a very distinct layer or septum of transverse 

 fibres is seen crossing from the one hemisphere of the cerebellum to the other. This 

 septum is best seen from behind, when the tracts which descend from the cerebrum 

 and from the corpora quadrigemina are taken away, for then its appearance (as in 

 Plate XX. fig. 1 . A. A.) much resembles the plates now to be described. 



As to those septa which I conceive have hitherto been neglected, the most remark- 

 able is that which forms a plane in the median line, resting with its edge upon the 

 last-named transverse septum, and extending its fibres directly backwards, so as to 

 form a striated leaf, separating the two great longitudinal tracts which pass between 

 the medulla oblongata and the thalami nervorum opticorum (Plate XX. fig. 1. b.). 



If we separate the corpus restiforme (meaning by that term the mass which passes 

 between the cerebellum and the medulla oblongata,) from the corpus olivare, we shall 

 find a layer of delicate fibres which constitute a pellicle much resembling the fibrous 

 layer, which might be peeled from the bark of the birch-tree, and this is a septum 

 (fig. 1. c. c). 



Another septum of the same kind intervenes between the two anterior corpora py- 

 ramidalia. So accurately are the extreme anterior fibres of this septum attached to 

 the corpora pyramidalia, that if we separate these bodies the fibres will alternately 

 adhere to the right and left column, so as to present an appearance as if there was 

 an actual commissure between them ; and authors have mistaken this, describing that, 

 which truly is a septum of separation, as a bond of union. And so on the back part 

 of the medulla oblongata, when we push aside the restiform bodies, or those columns 

 which have sometimes been called the posterior pyramidal bodies, and open the cen- 

 tral slit, we have the same appearance of minute commissures, which, however, is 

 only the separation of the fibres of the plate or septum ; and these fibres, instead of 

 running in a direction to be a lateral bond of union or commissure, run from before 

 backwards, and intervene between the longitudinal columns. 



These layers not only distinguish in a natural way the columns which are de- 

 scending from the cerebrum to form the spinal marrow, but they are necessary as 

 leading us to the true points of union between the longitudinal columns, where their 

 fibres actually decussate, and where these septa are deficient to permit the union. 



The Pons Varolii, or nodus cerebri, is undoubtedly an intricate part of the brain ; 

 but until this intricacy be explained, we can have no hope of making a correct ar- 

 rangement of the course of the filaments in the brain, and which pass through this 

 body. We shall therefore take it as a key to the composition of the brain. 



The pons has with seeming correctness been considered as the commissure of the 

 cerebellum. In this, its capacity of joining opposite parts, we have to notice its two 



