500 



THE NERVOUS SYSTEM 



organ ; it is therefore homologous with the central canal of the 

 spinal cord. At a slightly higher level, lateral to the grey matter 



FIG. 



5. A SECTION OF THE HUMAN BKAIN STEM AT THE 

 BORDER OF THE PONS VAROLII. 



LEVEL OF THE CEREBRAL 



Brcj, brachium conjunctivum ; cH, central tract of the tegmentura ; DBrcj, decussa- 

 tion of the brachium conjunctivum; DIV, decussation of the trochlearis nerve; Fpl, lat- 

 eral pontine bundle; Flp, posterior longitudinal fasciculus \fpLl, perforating fibres of 

 the lateral lemniscus ; fpPo, longitudinal pontine fibres; Fprd, predorsal bundle ; IVb, 

 descending root, and /Fc, point of exit of the trochlearis nerve; Leo, locus coeruleus; 

 LI, lateral lemniscus ; Lm, mesial lemniscus ; LmP, bundle from the fillet to the crusta ; 

 m, dorsal longitudinal fasciculus of Schtitz ; mPy, cortico -pontine tract ; Nca, superior 

 central nucleus ; NdR, dorsal nucleus of the raphe' ; NLl, nucleus of the lateral lemnis- 

 cus; Po, pons; Srt, substantia reticularis alba; Strc, middle, Sirp, deep, and Strs, su- 

 perficial layers of the pons ; Trs, rubro-spinal tract ; Tst, spino-tectal and thalamic tract ; 

 vH, ventral field of the tegmentum; F/F, fourth ventricle; Vlma, anterior medullary 

 velum ; F, crossed root of the trigeminus. Weigert's stain, x 3. (After Marburg.) 



surrounding the aqueduct and lying just mesial to the cerebral 

 root of the fifth nerve, is a small circular bundle of coarse nerve 

 fibres, the cerebral or descending root of the fourth cranial nerve. 



