CH. XLVIII.j SENSORY AREAS OF BRAIN 689 



a dotted circle. There are other centres concerned in speech, as 

 we shall see when considering the question of association fibres 

 (p. 695) ; but this is the centre for the muscular actions concerned 

 in speech. The discovery of this centre was the earliest feat in 

 the direction of cerebral localisation. It was discovered by a 

 French physician named Broca ; he noticed that patients who died 

 after haemorrhage in the brain, but who previous to death exhibited 

 a curious disorder of speech called aphasia, were found, after 

 death, to have the seat of the haemorrhage in this convolu- 

 tion. The convolution is generally called Broca's convolution. 

 Experiments on animals are useless in discovering the centre for 

 speech. Sherrington and Griinbaum found in the higher apes that 

 faradisation of the Broca area does not evoke vocalisation. 



The most curious fact about the speech-centre is that it is uni- 

 lateral ; it is situated only on the left side of the brain, except in 

 left-handed people, where it is on the right. We are thus left- 

 brained so far as the finer movements of the hand-muscles are con- 

 cerned, as in writing, and we are also left-brained in regard to speech, 

 an action intimately associated with writing. 



The visual area is in the occipital lobe. Removal of one occipi- 

 tal lobe in a monkey, or disease of that lobe in man, produces blind- 

 ness of the same side of each retina, or inability to see the opposite 

 half of the visual field. This is called hemianopsia ; the head and 

 eyes are turned to one side (conjugate deviation to the side of the 

 injury). Such an operation does not destroy vision in the central 

 portion (macula lutea) of either retina, because each macula sends 

 impulses to both sides of the brain. The macula is found only in 

 monkeys and man. Stimulation of one visual area leads to a subjec- 

 tive sensation apparently coming from the same halves of both retinas, 

 and also excites the motor cells (solitary cells, see p. 686) ; this pro- 

 duces conjugate deviation of head and eyes towards the opposite side 

 to that stimulated. 



The optic radiations consist of (1) sensory fibres from the optic tracts via the 

 external geniculate bodies ; (2) motor fibres to the centres for eye-movements ; and 

 (3) association fibres which are last developed. The last named link one convolu- 

 tion to others, and the two hemispheres together, and bring about association of 

 ideas of vision in both hemispheres, and with other sensations. A large collection 

 of such fibres runs horizontally through the grey matter. This white stripe is visible 

 to the naked eye ; it is the anatomical mark of the visuo-sensory cortex, and is called 

 the line of Gennari. The growth of the great parietal association centre pushes the 

 visuo-sensory area in man mainly on to the mesial surface of the hemisphere (see 

 area 4 in figs. 506, 507, p. 696). The visuo-psychic region (fig. 500) has no line of 

 Gennari, but possesses many pyramidal cells in its outer layers, which play the part 

 of association units where memory pictures are stored and visual sensations corre- 

 lated with those from other sense-organs ; the higher one ascends the animal scale, 

 the greater becomes the depth of this layer. The eye centre in the frontal lobe is 

 separated, as in the higher apes, by inexcitable grey matter from the rest of the 

 sensori-motor area. In the lower monkeys the anterior eye centre is not insulated 



2X 



