186 DISSECTION OP THE BRAIN. 



towards the surface, and the narrowed part or neck open and directed 

 backwards near the middle line. It consists of small, nucleated, and rami- 

 fied nerve cells. 1 



The nucleus is the yellowish white substance filling the capsule ; and 

 from it and the capsule issue transverse fibres, which unite the olivary 

 bodies across the middle line (fig. 58, g) : this uniting band is close below 

 the anterior median fissure. 



The fillet is a narrow band of fibres, which ascends from the olivary 

 body to the cerebrum (fig. 57). It is formed in part by longitudinal fibres 

 of the lateral tract which diverge to inclose the corpus olivare, and in part 

 by fibres derived from the sac. 



The restiform body (fig. 54, 3 ) is continuous inferiorly with the poste- 

 rior column of the cord. Superiorly it bends outwards to the cerebellum 

 without entering the pons. 



The posterior pyramid runs below into the posterior median column of 

 the cord, and is directed above along the floor of the fourth ventricle, 

 joining the fibres of the lateral tract of the same side. 



The decussation of the medulla oblongata (pyramids) (fig. 56, b) occu- 

 pies the anterior groove of the oblong medulla, at the distance of three- 

 quarters of an inch from the pons. It is about a quarter of an inch in 

 length, and is constructed by the crossing of three or four bundles of fibres 

 from each side. 



In this intercommunication the fibres are derived, according to Mr. 

 Clarke, from all the constituent parts of the half of the spinal cord of the 

 opposite side except from the anterior column ; but the fibres from the 

 lateral column of that side, blended with offsets from the contiguous gray 

 substance, form the chief portion of the decussation. 



Arched or commissural fibres of the medulla. In each half of the me- 

 dulla oblongata are fine transverse fibres, both on the exterior and in its 

 substance. At the middle line the fibres of opposite sides unite, and give 

 rise to a commissure between the halves (raphe). 



The superficial transverse fibres (fig. 58, s, and fig. 54), more marked 

 in some bodies, issue from the nucleus in the restiform body (Clarke), and 

 advance over the surface of the olivary and pyramidal bodies to the ante- 

 rior fissure, where they enter the half of the medulla of the same, and of 

 the opposite side. Below the olivary body they form oftentimes a disti.ict 

 band, the fibrae arciformes (fig. 54). 



The deep transverse fibres (fig. 58, h) are to be seen with the micro- 

 scope on thin hardened transverse sections ; they begin behind in nuclei 

 of the posterior pyramid and restiform body (Clarke), and penetrate be- 

 tween the longitudinal fibres, and through the corpus olivare, as they reach 

 forwards to enter the raphe in the middle line. 



The raphe (fig. 58, h and g), in which the arched fibres meet, occupies 

 the middle line of the medulla above the decussation of the pyramids, and 

 serves as the commissure between the halves of the medulla and the oli- 

 vary bodies. 



Gray matter of the medulla oblongata. In the medulla oblongata there 

 are the remains of the gray matter of the spinal cord ; and some special 

 deposits. Cross sections of the part when hardened would be required to 

 see them. 



1 The arrangement of the fibres in the sac is most complicated, and a fuller 

 account may be obtained by consulting Mr. Clarke's Paper in the Philosophical 

 Transactions for 1858. 



