THE VISUAL CENTRE. 703 



[Stimulation of the angular gyrus causes movements of the eyes to the 

 opposite side, with closure of the eyelids and contraction of the pupil. The eye- 

 balls were directed upwards or downwards according as the electrodes were applied 

 to the anterior or posterior limb of the angular gyrus (Ferrier). Stimulation of 

 the whole of the cortex of the occipital lobe, including its mesial and under 

 surfaces, causes conjugate deviation of the eyes to the opposite side, the direction 

 of movement varying with the position of the electrodes.] 



Mauthner denies the existence of cortical blindness, and believes that, after destruction of 

 the middle of the visual centre, the reason why the dog does not recognise the object with the 

 opposite eye is because, owing to there being only indirect vision, there is no distinct impression 

 on the retina. The position of the visual centre has been variously stated by different 

 observers. According to Ferrier, in the dog it lies in the occipital part of the III primary con- 

 volution, near the spot marked e, e, e, in fig. 483 ; according to his newer researches, in the 

 occipital lobe and gyrus angularis. 



Connection with the Retina. Munk asserts' that in dogs loth retina? are connected with each 

 visual cortical centre, and in such a manner that the greatest part of each retina is connected 

 with the opposite cortical centre, and only by its most external lateral marginal part with the 

 centre of the same side (fig. 487). If we imagine the surface of one retina to be projected upon 

 the centres, then the most external margin of the first is connected with the centre of the same 

 side, the inner margin of the retina with the inner area of the opposite centre, the upper margin 

 with the anterior area, and the lower marginal part of the retina with the posterior area of the 

 opposite side. The (shaded) middle of the centre corresponds to the position of direct vision of 

 the retina of the opposite side (compare 344). 



Stimulation of the visual centre in dogs causes movements of the eyes 

 towards the other side, sometimes with similar movements of the head and con- 

 traction of the pupils. If one eye be excised, from new-born dogs, the opposite 

 visual centre, after several months, is less developed (Munk). After extirpation 

 6f the visual centre in young dogs, the channels which connect it with the optic 

 nerve undergo degeneration (Monakow) ( 344). 



In monkeys, the centre occupies the occipital lobe. Unilateral destruction causes temporary 

 blindness of the halves of both retina?, i.e., hemianopia on the side of the injury. The visual 

 centre in pigeons (fig. 483, IV, where 1 is placed) lies somewhat behind and internal to the 

 highest curvature of the hemispheres (M'Kendrich, Ferrier, Musehold). The visual centre 

 in the frog lies in the optic lobe (Blaschko). 



[The visual path is along the optic nerve to the chiasma, where the fibres from the nasal half 

 of each retina cross to the optic tract, some of the fibres perhaps becoming connected with the 

 external corpora geniculata, and some with the pulvinar of the optic thalamus and corpora 

 quadrigemina, while the great mass sweeps backwards to the occipital lobes as the optic expan- 

 sion of Gratiolet. Destruction of this path behind the chiasma causes hemiopia or hemianopia, 

 and certain diseases of the occipital cortex cause a similar result. Perhaps, however, there is 

 another centre in the angular gyrus (and supra-marginal lobe), for in cases of word-blindness 

 disease has been found in these regions. Sometimes flashes of light or the appearance of a ball 

 of fire form the aura in epilepsy, and Hughlings Jackson thinks that discharging lesions of the 

 right occipital lobe cause coloured vision more frequently than on the left. ] 



2. The centre for hearing, or " auditory area," lies in the dog, according to 

 Ferrier, in the region of the second primary convolution at /, /, / (fig. 483, II), 

 while in the monkey and man it is in the first temporal or temporo-sphenoidal 

 gyrus (Ferrier's centre, No. 14). * Munk locates it in the same region. According 

 to Munk, destruction of the entire region causes deafness of the opposite ear, while 

 destruction of the middle shaded part alone causes "psychical deafness" (" Seelen- 

 taubheit"). Electrical irritation of the upper two-thirds of the superior temporal 

 convolution is followed by a reaction which closely resembles that produced by a 

 sudden fright, or that produced by a sudden unexpected noise. [There is a quick 

 retraction of the opposite ear, i.e., "pricking " of the ear as if toward the supposed 

 origin of the sound, combined generally with turning of the head and eyes to that 

 side, and dilatation of the pupil.] Ferrier locates the centre for hearing in the 

 monkey in the superior temporo-sphenoidal convolution, and he finds that, when the 

 centres on both sides are extirpated, the animal is absolutely deaf; it takes no 

 cognisance of a pistol fired in its neighbourhood. [From his experiments on monkeys, 



