54 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



unpaired in appearance, or it ma\' consist of a pair of lateral 

 lobes or hemispheres separated b}' a median portion or vermis, 

 terminating in a small lobe, the valve of Vieussens, which roofs 

 in the fourth ventricle in front. 



The myelencephalon or medulla oblongata is the cranial 

 extension of the spinal cord, presenting behind but slight dif- 

 ferences from that structure. In front it widens, while its roof 

 thins out and becomes epithelial and folded, to form the poste- 

 rior choroid plexus for the underlying fourth ventricle. This 

 region of thinning is known, from its shape, as the fossa rhom- 

 boidAlis. It is bounded in front by the valve of Vieussens, and 

 on either side by the diverging dorsal columns of the cord 

 (p. 44), which are frequently subdivided into a median fasciculus 

 gracilis (Column of GoU) and a more lateral fasciculus cuneatus 

 (Burdach's Column). (Fig. 46.) Each dorsal column receives 

 in front fibres from the lateral column, the whole forming an 

 enlargement, corpus restiforme, on either side, from which the 

 posterior pedxmcles of the cerebellum pass forward and upward 

 into the metencephalon. On the ventral surface are the anterior 

 ends of the ventral columns (p. 44), here known as the pyra- 

 mids. These can be followed forward until they pass into the 

 crura cerebri already mentioned. In the higher vertebrates the 

 anterior ends of the pyramids are crossed by transverse bundles, 

 forming the pons Varolii, which act as commissures connecting 

 the two halves of the cerebellum. The medulla oblongata is 

 further noticeable since it gives rise to the greater part of the 

 cranial nerves. 



The various parts of the brain are connected bv longitu- 

 dinal fibre tracts and by transverse fibres or commissures. 

 Some of these have already been mentioned, and some others 

 may be noticed here. The chief longitudinal tracts are those of 

 the pyramids, which may be followed through the crura cerebri 

 to the corpus striatum. Some of the fibres of the lateral 

 column and a part of those of the dorsal column enter the 

 cerebellum through the posterior peduncles of the cerebellum, 

 while the majority from these columns end in the medulla. 

 From the cerebellum, fibres extend forward into the mid brain 

 through two bands of tissue known as the anterior peduncles of 



