802 



THE NERVOUS SYSTEM 



These nuclei are the groups of nerve cell-bodies about which the ascending or sensory 

 axones of the respective fasciculi terminate or where the sensory impulses are transferred to a 

 second neurone in their course to the structures of the encephalon. These cell-bodies in their 

 turn give off axones which immediately cross the mid-line and assume a more ventral position, 

 contributing largely to the lemniscus or fillet of the opposite side, and thus such axones are the 

 encephahc continuation of the central sensory pathway conveying impulses from the periphery 

 of one side of the body to the opposite side of the cerebrum. The crossing of these axonesiis 

 known as the decussation of the lemnisci. 



Fig. 631. — Dorsal Aspect of Medulla Oblongata and Mesencephalon, Showing the 

 Floor of the Fourth Ventricle (Rhomboid Fossa). (Modified from Spalteholz.) 



stria medullaris of thalamus v j^^^^^^j ^^p^^^^ 



Habenular commissuf e ■ 



Trigonum habenuls • 



/ ^\ 



*»*■ 



Caudate nucleus 



Epiphysis 



Medial genicu- 

 late body 



Lateral genicu-^ 

 late body ~ 



Brachium of inferior 

 quadrigeminate body 



Cerebral peduncle 

 Anterior medullary velum 

 Brachium conjunctivum 



Brachium of pons 



Restiform body 



-- Tffinia chorioidea 



Stria terminalis 

 of thalamus 



Pulvinar cf 

 "" thalamus 



Quadrigeminate bodies 



Trochlear nerve 



Calamus scriptorius 



Funiculus gracilis 



Funiculus cuneatus 



Lingula cerebelli 



StrisB medullares 

 acustici 



- Trigonum of vagus (ala cinerea) 



Nucleus of fasciculus cuneatus 

 Obex 



Nucleus of fasciculus gracilis Cclava) 



Posterior median fissure 



Lateral funiculus 



-Posterior intermediate sulcus 



With the termination of tlie dorsal funiculi 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 ven- 

 tricle. The floor of the fourth ventricle, which corresponds to the floor of the 

 central canal, is considerably widened into two lateral recesses opposite the junc- 

 tion of the inferior and middle cerebellar peduncles of either side, and, being 

 pointed at both its superior and inferior extremities, it is rhomboidal in shape and 

 thus is the rhomboid fossa. The pia mater of the spinal cord is maintained across 

 the tip of the calamus scriptorius to form the obex, a small, semilunar lamina 

 roofing over the immediate; opening of the central canal. The obex carries a few 

 medullated commissural fibres. 



