CONDUCTING PATHS IN MESENCEPHALON. 915 



traction of the flexors of the elbow, and inhibition of the right extensor 

 cruris, with contraction of the hamstring muscles ; while, on the 

 contrary, there ensues inhibition of the flexors of the right elbow with 

 contraction of the triceps brachii, together with inhibition of the right 

 hamstring muscles, together with contraction of the right extensor cruris. 

 In this reflex there is also bending of the neck and turning of the head 

 toward the left side, somewhat as if to meet the protracted and raised 

 left arm. 



Besides reflexes obviously associated with an "act," as the above 

 are with quadrupedal progression, others can be elicited when the 

 mesencephalon is the highest part of the encephalon remaining, some of 

 them less easy of interpretation. The following may be mentioned 

 as fairly significant. Irritation of the pinna in the dog and cat evokes 

 with the progression reflex a turning of the head and neck away 

 from (not as with fore-foot stimuli toward) the side stimulated. This 

 movement frequently culminates in the shaking of the head from side to 

 side as though to shake the ears. 1 Mechanical stimuli to the nostrils 

 provokes a fit of sneezing, but not, as far as I have seen, of coughing. 



Tracts of conduction. Paths that ascend in the mesencephalon and 

 partly end in it, lie in the fillet, and these come partly from the ventro-lateral 

 column of the spinal cord, partly from the end-nuclei of the auditory nerve. 

 A large path from the cerebellar hemisphere crosses the midsagittal plane to 

 enter and in part end in the red nucleus of the tegmental region. Severance of 

 the tegmental region and fillet on one side of the midline produces complete 

 anaesthesia and analgesia of the skin of the contralateral half of the body, the 

 analgesia, at first total in degree, gradually subsides to partial. The posterior 

 longitudinal bundles running up and down the region seem mainly commissural 

 and associational between the ocular nuclei. 



The main descending paths of the region are the mesencephalo-spinal, a 

 crossed path traceable from the anterior and posterior corpora quadrigemina 

 into the ventral part of the opposite ventro-lateral spinal column ; 2 I would 

 interpret this as a projection system belonging to vision. Then there are in 

 the mammal, the cerebral descending paths, fronto-pontine, temporo-pontine, 

 and pyramidal. These in their descent are seen to give fibres to the sub- 

 stantia nigra just dorsal to them, and from the pyramidal pass fibres, probably 

 indirectly, to the motor root cell system, represented here by the oculo-motorius 

 and trochlearis nuclei ; some fibres pass to the crossed and some to the homony- 

 mous nerves. Cajal 3 showed that efferent fibres pass out of this region into 

 the retina by way of the optic tracts and nerves, and these have been traced by 

 the degeneration method from the optic lobe of the bird into the crossed optic 

 nerve. 4 Conversely from the ganglion cell layer of the retina of the bird 

 passes a path into the " optic nuclei," dorsal and ventral, of the surface layer 

 of the crossed optic lobe. 5 In some birds (e.g. owl) the connection is probably 

 partly uncrossed ; in mammals it seems to be always partly uncrossed, 6 and the 

 ratio of the size of the uncrossed to the crossed portions is particularly large in 



1 Christian! notes this reflex, "Zur Physiologic d. Gehirns," Berlin, 1885. 



2 H. Held, Neurol. CentralU., Leipzig, 1890, Bd. ix. ; Arch.f. Anat. u. Entwcklngsgesch. , 

 Leipzig, 1893; R. Boyce, Phil. Trans., London, 1895 (communicated 1894); Ferrier and 

 W. A. Turner, ibid., 1894 ; Miinzer and Wiener (in bird), Monatschr. f. PyscMat. u. Neurol., 

 Berlin, 1898, Bd. iii.; Boyce and Warrington, Proc. Roy. S'oc. London, 1898, vol. Ixiv. 



3 Anat. Anz., Jena, 1889 ; Cellule, Lierre et Louvain, 1893, tome ix. 



4 Jelgersma, Neurol. CentralU., Leipzig, 1895, S. 290 ; Wallenberg, ibid., 1898 ; Boyce 

 and Warrington, Proc. Eoy. Soc. London, 1898, vol. Ixiv.; and Fourth Internal. Cong. 

 Physiol., Cambridge, Sept. 1898. 



5 Miinzer and Wiener, Monatschr. f. Pyschiat. u. Neurol., Berlin, 1898, Bd. iii. S. 379. 



6 Monakow, Arch.f. Psychiat., Berlin, 1892, Bd. xxiv. 



