384 



PHYSIOLOGY 



CHAP. 



account many anatomists give the name brain-stem to those parts 

 of the medulla and pons which are the direct continuation of the 

 spinal cord (Fig. 202). 



The pyramidal tracts as we saw in the last chapter decussate 



Fin. 202. View of medulla oblongata, pons Varolii, crura cerebri, and central parts of encephalon 

 from right side. (Allen Thomson.) The corpus striatum and thalamus options have been 

 preserved in connection with the central lobe and crura cerebri, while the remainder of the 

 cerebrum has been removed. St, upper surface of corpus striatum ; Th, back part of tha- 

 lamus options (pulvinar) ; C, placed on the middle of the five or six convolutions constituting 

 the central lobe or island of Reil, the cerebral substance being removed from its circumference ; 

 Sy, fissure of Sylvius, from which these convolutions radiate, and in which are seen the white 

 striae of the olfactory tract ; I, the olfactory tract divided and hanging down from the groove 

 in the convolution which lodges it ; II, optic nerves a little way in front of the chiasma ; 

 a, right corpus albicans with tuber cinereum and infundibnlum in front of it ; h, hypophysis 

 or pituitary body ; e, external, i, internal corpus geniculatum at back part of optic tract ; P, 

 peduncle or cms of cerebrum; III, right oculo-motor nerve; p, pineal gland; q, corpora 

 quadrigemina ; IV, trochlear nerve rising from v, valve of Vieussens. The following numbers 

 and letters refer chiefly to parts in connection with medulla oblongata and pons : V, on pons 

 Varolii above right nervus trigeminus ; s, superior, m, middle, in, inferior peduncle of cere- 

 bellum cut short ; VI, 6th nerve ; VII, facial nerve ; VIII, auditory nerve ; IX, glosso- 

 pharyngeal nerve; X, opposite cut end of pneumogastric nerve; XI, uppermost fibres of 

 spinal accessory nerve; XII, hypoglossal nerve; pa, pyramid; o, olive; ar, arciform fibres ; 

 r, restiform body ; tr, tubercle of Rolando ; fa, ventral, cp, dorsal, d, lateral columns of 

 spinal cord ; CI, Ci, ventral and dorsal roots of 1st cervical nerve. 



in the lower part of the bulb, turning sharply ventralwards to 

 form the ventral or anterior pyramids. By this decussation (Figs. 

 203 and 204) the ventral horns become detached and separated 

 from the rest of the grey matter. 



