678 FUNCTIONS OF THE CEKEBRDM. [CH. XLVUI. 



preceding figure the letterpress underneath them should be carefully 

 consulted. 



The marginal convolution was first investigated by Schafer and 

 Horsley, and to them belongs the credit of discovering the centre 

 for the trunk nuiscles. If one marginal convolution is removed 

 in an animal, there is much more marked pai*alysis of the opposite 

 limbs than of the trunk ; if the two marginal convolutions are 

 removed, there is very complete paralysis of the trunk as well as of 

 the limbs. In cases of hemiplegia in man, there is usually very little 

 paralysis of the trunk muscles. It is the muscles which act nor- 

 mally unilaterally that are most paralysed. The muscles of the 

 trunk always normally move bilaterally ; thus we use both sides of 

 our chest in breathing ; both sets of back muscles in maintaining 

 an erect position, and so on. The spinal- centres of the muscles 

 of the two sides are, no doubt, connected by commissural fibres, 

 and therefore can be affected from both sides of the brain. 



The Speech centre. This is surrounded by a dotted circle in 

 fig. 501. There are other centres concerned in speech besides 

 this, but this is the centre for the muscular actions concemed 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 exhi- 

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

 death, to have the seat of the haemorrhage in this convolution. The 

 convolution is generally called Broca's convolution. Experiments on 

 animals are obviously useless in discovering the centre for speech. 



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

 unilateral ; 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 concerned, and we are also left-brained in regard to speech, an 

 action which is apparently bilateral. (See also p. 738.) 



The Sensory areas of the Human Brain. These are much 

 less accurately mapped out than the motor areas. They are in 

 part coincident with the so-called motor areas, and in part situated 

 behind these. 



The visual area is situated in the occipital lobe, and the angxilar 

 gyrus. Extirpation of one occipital lobe in an animal, or disease 

 of that lobe in man, produces blindness of the same side of each 

 retina ; this condition is called hemianopsia. 



If, for instance, the right occipital lobe is removed, the result 

 is blindness of the temporal half of the right retina, and the nasal 

 half of the left retina, leading to an inability to see things in the 



