9 i2 THE MESENCEPHALON. 



ments evoked by stimuli to the skin. The latency may be increased 

 eight-fold, or the reaction may even be suppressed altogether. This 

 " inhibition " holds good for " pathic " stimuli, but not for pure 

 " tactile." l In connection with this inhibition of spinal reflexes and 

 the close ties between the mesencephalic roof and the retina?, it is 

 interesting that in the blinded frog the croak reflex can be elicited with 

 machine-like regularity. 2 



In birds, 3 excitation of the optic lobes evokes contralateral pupillar 

 dilatation, spreading, raising, and flapping of the contralateral wing, and 

 movement of both legs. Faradic stimulation of an anterior corpus 

 quadrigeminum in the rabbit and in carnivora i produces dilatation of 

 the contralateral, and later of the homonymous pupil, conjugate devia- 

 tion of the eyes upward and to the opposite side, retraction of the ears 

 and angles of the mouth. To these effects are added pricking of 

 the crossed ear, and vocalisation if the electrodes are applied to the 

 posterior instead of the anterior corpus. In the dog, 5 but not in the 

 rabbit, 6 it has been possible to elicit different co-ordinated eye movements 

 from different points of the anterior corpora ; from the sulcus between 

 them, return of the divergent bulbs to parallelism without any change 

 in the pupils ; from a posterior point, elevation of the eyes with dilatation 

 of both pupils; further back still, convergence with contraction of both 

 pupils. After sagittal section in the median plane, the excitation 

 affects only the homonymous eyeball. In all the conjugate reactions, 

 the contraction of one muscle is accompanied by relaxation of the tonus 

 of its antagonist. But the eyeball movements elicited are not always 

 conjugate ; and sometimes one only is elicitable from the whole of both 

 the anterior corpora. 7 In the monkey, Ferrier 8 discovered reactions 

 similar to those of the dog, with the addition of wide opening of the 

 palpebral fissures, elevation of the eyebrows, and turning of the whole 

 head in the direction of the gaze, i.e. away from the side of excitation. 

 Excitation of the posterior corpora in the monkey produces vocalisation, 

 the sounds ranging among many varieties of cries. 



Ablation. — The frog, after removal of all parts in front of the 

 mesencephalon, but with the latter intact, continues to exhibit 

 occasional movements that are " spontaneous." 9 It swims and springs 

 co-ordinately, but it no longer reacts by climbing to the tilted board. 

 The condition of the animal resembles torpor ; let down gradually into 

 water, it floats without stirring, although it still can swim. Probably 

 vision and smell are gone. Croaking is easily elicited. On stroking 

 the flanks, not only is croaking elicited, but the animal gradually rises 

 on its four limbs with curved back and muscles in tonic contraction. 

 It remains so for many minutes after the stimulus to the flank skin has 

 been discontinued, even for half an hour. 10 Destruction of the mesen- 

 cephalon renders the croak reflex difficult to elicit, and altogether 

 abolishes the tonic reflexes, though the latter are still possible in the 



1 Setschenow and Paschutin, "Neue Versuche," Berlin, 1865 ; and Setschenow, Mem. 

 Acad. imp. d. sc. de St. Petersbourg, tome xx. p. 357 ; and "Ueber d. elekt. u. chemisch. 

 Reizung. d. sensibl. Riickenmarksnerven," Berlin, 1868. 



2 Langendorff, Arch. f. Physiol., Leipzig, 1877. 



3 Ferrier, "Functions of the Brain," London, 1876. 4 Ferrier, op. tit. 



5 Adaniuk, Arch. f. Ophth., Leipzig, 1872, Bd. xviii. S. 153. 



6 Knoll, Sitzungsb. d. k. Akad. d. Wisscnsch., Wien, July 1886, S. 239. 



7 Knoll, ibid. 8 Op. tit. 



9 Volkmann, art. " Gehirn," Wagner's " Handworterlmch," Braunschweig, 1844, S. 582. 

 10 Verworn, Arch. f. d. gcs. Physiol., Bonn, 1897, Bd. lxv. S. 63. 



