786 



THE NERVOUS SYSTEM 



side and thus such axones are the encephalic continuation of the centra sensory 

 pathway conveying impulses from the periphery of one side of the body to the 

 opposite side of the cerebrum. The crossing of these axones is known as the decus- 

 sation of the lemnisci. 



With the termination of the dorsal fumculi and the ventral course of the fibres of 

 the lemnisci in their decussation, the central canal of the spinal cord loses its roof 

 of nervous tissue in the medulla and comes to the surface as the fourth ventricle. 



FIG. 583. DIAGRAM OF THE DECDSSATION OF THE LEMNISCI. 



K^~,^\-- PYRAMID 



. . LEMNISCUS (FILLET) 



INFERIOR OLIVARY NUCLEUS 



COCHLKJUIIS t 



XUCLBUS Mill TUM-rr.-s ..,//. 



.v.i /./.- r it a, KM ix I 



RBSTIfORH BODY 



L - KUCLEl'S VIWlltrLARIS 



. DECVSSA raw OF LEMNISCI 



XrCLKTS AM> TXM-Tfx XflHALIS 

 TR1GKMIM 



INTERNAL ARCUATE FIBRES 



JfVCLKI'X 1-TXICULI CUNEATI 

 NUCLEUS FUNICULI GRACILIS 



FASricrLl'X CfXEATrfS (fioll) 

 - FASCICULUS GRACILIS (Burdach) 



The floor of the fourth ventricle, which corresponds to the floor of the central canal, 

 is considerably widened into two lateral recesses opposite the junction of the cere- 

 bellar peduncles of either side, and, being pointed at both its superior and inferior 

 extremities, it is rhomboidal in shape and thus is the fossa rhomboidea. The pia 

 mater of the spinal cord is maintained across the tip of the calamus scriptorius to 

 form the obex, a small, non-nervous, semilunar lamina roofing over the immediate 

 opening of the central canal. 



THE PONS 



The pans (Varolii) is, for the most part, a great commissure or 'bridge' of white 

 substance coursing alaout the ventral aspect of the brain-stem, and connecting the 

 cerebellar hemisphere of one side with that of the other. In addition it contains 

 fibres passing both to and from the structures of the brain-stem and the grey sub- 

 stance of the cerebellum. Each of its lateral halves forms the middle of the three 

 cerebellar peduncles, the brachium pontis. 



In size it naturally varies directly with the development of the cerebellum, both 

 in a given animal and relatively throughout the animal series. In man it attains its 



