THE VISUAL AREA. 759 



the superior corpora quadrigemina, but none directly to the thalamus. From 

 the lateral geniculate body a stout bundle passes to the pulvinar thalami, but 

 the fibres for the most part pass through this to join the optic radiations of 

 Gratiolet and to pass in these to the margins of the calcarine fissure. Other 

 fibres of the optic radiations, the origin of which is somewhat uncertain, spread 

 out into the whole of the cuneus and lingual lobule, and extend over the 

 posterior pole of the hemisphere to the basal surface (occipito-temporal region). 

 No fibres of the optic radiations go to the gyrus angularis, which is therefore 

 entirely excluded by Flechsig from the visual area of the cortex. 



Steiner found that in the cat and rabbit the visual area does not begin to 

 respond to electrical excitation until about the fourteenth or fifteenth day 

 after birth, whereas response is obtained from the Eolandic area about the 

 tenth day. In the dog the response of the visual area is obtained still later 

 (thirtieth to fortieth day). It is noteworthy, in connection with this observa- 

 tion, that puppies do not appear to recognise objects by sight until after about 

 the twenty-third day after birth, and do not appear to recognise objects the 

 images of which fall on the periphery of the retina until about the thirty-fourth 

 day. 1 It would appear from the observations of Kaehlmann upon infants, that 

 these do not recognise objects the images of which fall upon the centre of the 

 retina until the fifth week after birth, and those which fall upon the periphery 

 of the retina not until the fifth month. 



The view has been taken by Ferrier, as the result of his experiments, 

 that while the occipital lobe may be concerned with visual perceptions 

 due to nervous impulses derived from the homonymous part of the same 

 retina, the angular gyrus is especially connected with central vision of the 

 opposite eye ; and this view has been adopted by several other clinicians. 

 His statement that blindness of the opposite eye, of a transient character, 

 is caused by destruction of that gyrus, has, however, been confirmed only 

 by one other observer, namely, Lannegrace, 2 and is denied by all others 

 who have experimented upon the subject ; nor has it been shown in 

 any animals with bilateral vision that lesion of one hemisphere only is 

 productive of complete blindness of the opposite eye or of loss of central 

 vision. It is true that cases of amblyopia, involving one eye only, 

 accompanying cerebral lesions, are not unknown clinically, but they are 

 not specially connected with lesions localised to the angular gyrus, and 

 the symptom is always combined with anaesthesia of the globe. 3 It 

 appears certain that monocular defects of vision, when not due to 

 injury of the optic nerve, are caused by a disturbance in the nutrition of 

 the retinal elements (perhaps produced indirectly by an alteration in the 

 blood supply), and are on a par with the defects in taste and smell 

 which are brought about by lesions of the trigeminal nerve. 4 The 

 monocular defects which are observed in cases of functional or hysterical 

 amblyopia may, according to Knies, 5 be due to vasomotor disturbances 

 (vaso-dilatation) affecting the conduction of the fibres of the optic nerve, 

 where these pass through the foramen opticum. That the disturbance 

 in these cases is not retinal, is shown by the fact that the ophthal- 

 moscopic image is normal. 



In progressive pathological lesions of the occipital lobe, loss or affection of 



1 Steiner, Sitzungsb. d. k. Akad. d. Wissensch., Wien, 1895, S. 303. 



2 Arch, de m6d. exptr. et d'anat. path., Paris, 1889. 



3 v. Bechterew, Neurol. CentralbL, Leipzig, 1894, S. 252 and 297. 



4 Defective vision in the opposite eye has been experimentally obtained by section of 

 the ascending root of the fifth nerve in the medulla oblongata, and is ascribed by v. Bech- 

 terew to vasomotor and trophic disturbance of the retinal elements (loc. tit.}. 



5 Neurol. CentralbL, Leipzig, 1893, S. 570. 



