1888.] On the Latency Periods of the Ocular Muscles. 411 



III. "A Comparison of the Latency Periods of the Ocular 

 Muscles on Excitation of the Frontal and Occipito- 

 Temporal Regions of the Brain." By E. A. Schafer, 

 F.R.S., Jodrell Professor of Physiology in University 

 College, London. Received February 13, 1888. 



Conjugate deviation of the eyes to the opposite side is produced by 

 excitation of entirely different regions of the cerebral cortex. The 

 parts which when electrically excited produce this movement are : 

 (1) An area in the frontal region of the hemisphere which is included 

 in the motor or psychomotor zone of authors ;* (2) the superior 

 temporal gyrus ; (3) the upper end of the middle temporal gyrus ; 

 (4) the posterior limb of the angular gyrus ; (5) the whole cortex of 

 the occipital lobe including its mesial and under surfaces ; (6) the 

 quadrate lobule. 



Of these parts, excitation of which produces this result (conjugate 

 deviation of the eyes to the opposite side), one, viz., the frontal area, 

 is distinguished from the rest by the fact that its removal produces 

 paralysis of that movement. This fact has been seized upon by 

 Ferrier as indicating an important functional difference, the move- 

 ments in the one case being probably caused by the direct action of 

 this part of the cortex upon the centre of origin of the nerves to the 

 ocular muscles ; but in all other cases by indirect action, the move- 

 ment when, e.g., the visual or auditory region is stimulated being the 

 result of visual or auditory impressions (subjective sensations) being 

 provoked in the brain by the excitation, and these impressions 

 producing indirectly the action in question. Others have supported 

 the view that in all cases the movement is the result of the setting 

 up of subjective sensations, but that in the case of the frontal area 

 these are tactile or are connected with the muscular sense. 



It seemed to me that light would be thrown upon the question if 

 the period of latent stimulation of the ocular muscles were accurately 

 determined under exactly the same conditions for the frontal and 

 posterior (temporal and occipital) areas respectively. The result of 

 this determination, which I have made in a number of monkeys, is 

 to show that the latent period is longer by some hundredths of a 

 second in the case of stimulation of the occipital lobe,- or of the 

 superior temporal gyrus than when the frontal area is stimulated ; 

 thus indicating that in the former case the nervous impulses must be 

 transmitted through at least one more nerve centre than in the 

 latter. 



* For the exact limits of this area see a piper, " Ueber die motorischen Bindenf- 

 centren des Affengehirtis," in 'Beitrage zur Physiologic, C. Ludwig gewidmet,' 1886. 



