MESENCEPHAL ON. 



MESENCEPHALON OK EEGION OF THE CORPORA QUADRIGEMINA. 



911 



An afferent root cell system has not been identified in this region, 

 though there are reasons for thinking it exists in connection with sensory 

 organs in the ocular muscles. 1 



The efferent root cell system comprises the motor cells innervating 

 all the eye muscles, except the lateral rectus and the dilatator 

 pupillse. Some of the cells of the oculo-motorius are crossed, 2 that is, 

 contribute the nerve fibre process to the nerve trunk of the crossed 

 side ; but to which of the eye muscles the crossed fibres pass is not yet 

 ascertained. The fibres of the trochlearis decussate in their peripheral 

 course on the roof, the mesencephalon. Whether the decussatioii is 

 total is not yet ascertained. The smaller root cells of the oculo- 

 motorius send their nerve fibres to the ciliary ganglion, whence, as from 

 a sympathetic ganglion, fresh nerve cells emit fibres (but in this case, 

 unlike the sympathetic elsewhere, myelinated) to the ciliary muscle and 

 constrictor pupillae. Among the units of the motor root cell system 

 are perhaps to be reckoned those that supply the upper facial muscles, 

 frontalis, corrugator, and orbicularis palpebrarum ; 3 the nerve fibres 

 from these root cells do not, however, emerge from the central nervous 

 system in the mesencephalon, but pass down to join those which inner- 

 vate other facial muscles, and leave the metencephalon in the facial 

 nerve. 



The distinction between roof and floor in the mesencephalic region, 

 though clear enough, does not so lend itself to experimental opportunity 

 as in the parts of the brain previously dealt with. The nervous masses 

 of the roof form in mammals the corpora quadrigemina, the hinder pair 

 of which are the enlarged equivalents of two comparatively small 

 nuclei, occupying only a small hinder edge of the lobi optici of fish and 

 Sauropsida. The relatively huge size of the front pair in reptiles arid 

 birds harmonises with the preponderant role played by vision in those 

 animals. 



Excitation of the mesencephalic roof succeeds under stimuli of all 

 kinds, electrical, mechanical, thermal, and chemical ; electrical stimuli 

 are most efficient. In fish, 4 the tail is drawn toward the side excited, 

 and the dorsal and anal fin are spread and inclined towards that side ; 

 the pectoral fin is spread, and there is movement of the eyeball of the 

 same side. In the frog the head is thrown to the opposite side and 

 upward, and croaking may be elicited. The heart is usually inhibited, 

 but not strongly. 5 If telencephalon and diencephalon have been pre- 

 viously ablated, weak and prolonged excitation of the mesencephalic roof 

 by sodium chloride increases (Setschenow) 6 greatly the latency of move- 



1 Sherrington, "Proc. Phys. Soc.," June 1894, Journ. Physiol., Cambridge and London, 

 1894, vol. xvi. ; Proc. Roy. Soc. London, 1897, vol. Ixi. p. 247 ; 1898, vol. Ixiii. 



2 Perlia, Arch. f. Ophth., Leipzig, Bd. xxxv. ; Spitzka, Trans. Phila. Neurol. Soc., 

 1885; Edinger, " Vorlesungen ueber d. nerv. Centralorg.," 1889; Kolliker, Sitzungsb. 

 d. phys.-med. Gesellsch. zu Wilrzburg, 1892; van Gehuchten, Cellule, Lierre et Louvain, 

 1892, tome viii. ; Cramer, " Beitr. z. fein. Anat. d. Briicke," Jena, 1894; van Gehuchten, 

 Rev. neurol., Paris, 1898, tome vi. 



3 Mendel, Neurol. Centralbl., Leipzig, 1887, S. 537 ; H. Tooth and W. A. Turner, 

 Brain, London, 1891, vol. xiv. p. 490 ; Oppenheim, Arch. f. Psychiat., Berlin, 1892, 

 Bd. xxiv. S. 758 ; Muratoff, Neurol. Centralbl., Leipzig, 1892, Bd. x. S. 514 ; W. A. 

 Turner and Bulloch, Brain, London, 1894, vol. xvii. 



4 Ferrier, "Functions of the Brain," London, 1876. 



3 Wilson, Journ. Physiol., Cambridge and London, 1890, vol. xi. 

 6 " Physiologische Studien," Berlin. 1863. 



