vin OCULAK MOVEMENTS 401 



deviation of both eyes towards the side of the lesion owing to 

 the predominance of the non-paralysed antagonist muscles; 

 convergence, on the contrary, is unaffected, because the angular 

 gyrus and the paths from one hemisphere to the internal rccti 

 of both eyes are intact. The conjugate deviation may be com- 

 pensated in time ; but, if the lesion is sufficiently extensive and 

 complete, the inability to turn the eyes towards the side opposite 

 that of the lesion persists. 



Voluntary cortical impulses may proceed directly from the 

 gyrus angularis, or be transmitted indirectly to the gyrus 

 angularis from the visual area or other regions of the cortex 

 (frontal lobe) by the association fibres which unite the different 

 parts of the cerebral cortex into a single organ. 



Involuntary reflex movements of the eyes are excited princi- 

 pally through the optic nerves, and, according to Bernheimer, the 

 impulses reach the cortical centres directly, without interposition 

 of the grey matter of the anterior corpora quadrigemina, as was 

 formerly assumed by Meynert and Kolliker. 



The nature of the anatomical and physiological basis of the 

 exceedingly delicate, rapid, and certain co-ordination of the eye- 

 movements which subserve directive adaptation is a very difficult 

 problem which has not yet been adequately solved. It may be 

 stated generally that it does not depend exclusively upon a 

 sensation of innervation preceding the volitional impulse that 

 changes the direction of the visual axes, as assumed by Meynert, 

 Bain, Helmholtz, and Wundt ; nor upon the peripheral kinaesthetic 

 sensations which accompany the contraction of the eye-muscles, 

 as maintained by James and Miinsterberg ; but it is also due to 

 the changes in position of the objects in the field of vision which 

 accompany the displacements of the visual axis. According to 

 Helmholtz we constantly use this displacement of objects as a 

 control of the proper relation between the volitional impulses 

 and their effects. It is obviously of greater importance in 

 consciousness than the obscure kinaesthetic sensations of innerva- 

 tion which precede or accompany the eye-movements, and is in 

 itself adequate to account for the rapidity of directive adaptation 

 in the external muscular system, which is associated and co- 

 ordinated with the accommodation and convergence of the internal 

 muscles of the eyes. 



IV. Owing to this central association and the co-ordination of 

 their movements, the two eyes constitute a single binocular 

 instrument, which Hering termed the " double eye." The two 

 eyes are habitually in such positions that the points towards 

 which the two visual axes are directed, and which form images 

 upon the two foveae centrales of the retina, induce single vision 

 that is, the central fusion of the two images into one. 



The fundamental conception underlying single vision with 



VOL. IV 2 D 



