42 MORPHOLOGICAL TYPES 



There is also an (anterior) transverse commissure in the 

 lamina terminalis, but on the whole there is little inter- 

 connexion between the two sides of the brain. 



The myelencephalon, or medulla oblongata, passes back 

 gradually into the spinal cord. This is a tube with thick walls 

 and a small central cavity, continuous of course with that of the 

 brain. The nerve-cells are grouped round the centre of the 



he pcop. Rf. 

 I Li i 



oU- . 



CP3. 



vi. 



>b 



Fig. 22. — Scyllium : median view of a longitudinal section through 

 the brain. 



The various regions of the brain are separated by broken lines across 

 the central cavity, and indicated by the letters : T, telencephalon (end- 

 brain) ; D, diencephalon (or thalamencephalon, between-brain) ; Ms, 

 mesencephalon (midbrain) ; Mt, metencephalon (anterior part of hind- 

 brain) ; My, myelencephalon (posterior part of hindbrain) ; ac, anterior 

 commissure ; c, cerebellum ; cp 3 and 4, choroid plexus of the third and 

 fourth ventricle ; he, habenular commissure ; i, infundibulum ; //, lamina 

 terminalis ; mo, medulla oblongata ; oc, optic chiasma ; ol, olfactory lobe ; 

 op, optic lobe ; pb, pituitary body ; pc, posterior commissure ; pe, pineal 

 stalk ; Rf, Reissner's fibre ; rn, recessus neuroporis ; sc, spinal cord ; 

 v 3 and 4, cavity of the third and fourth ventricle ; vt, velum transversum. 

 (Partly after Nicholls.) 



cord, and form the " grey matter." Outside them and 

 occupying the remaining space are the ascending and descend- 

 ing tracts of nerve-fibres, provided with medullary sheaths, 

 and forming the " white matter." This arrangement of 

 central grey matter and peripheral white matter holds also in 

 the brain. Only in the cerebellum and in the optic lobes are 

 there some superficial nerve-cells ; i.e. grey matter outside 

 white. 



