FUNCTIONS OF THE BRAIN 



965 



lated than when the white hbres are directly excited, (b) Morphine 

 greatly increases the period of delay for stimluation of the cortex, and 

 at the same time renders the resulting contractions more prolonged 

 than normal, while the results of direct stimulation 01 the white fibres 

 are much less, if at all, affected, (c) Stimulation of the grey matter, 

 when separated from the subjacent white matter by the knife, but left 

 in position, is without effect unless the strength of stimulus be increased, 

 although twigs of the current ought, of course, to pass into the corona 



radiata as easily as before. 



Jlp 



Perfectly definite movements 

 can, however, be excited or 

 inhibited by stimulating de- 

 finite spots in the corona radi- 

 ata, and even in the internal 

 capsule. This simply means 

 that in these positions the 

 fibres representing these move- 

 ments are not yet intermingled 

 with fibres representing other 

 movements. 



Sensory Areas Visual Cen- 

 tres. In the occpital lobe in 

 animals an area of consider- 

 able extent has been found, 

 destruction of which causes 

 hemianopia (p. 923). Thus, 

 if the right occipital cortex is 

 destroyed, the right halves 

 of the two retinae are para- 

 lyzed, and the left half of the 

 field of vision is a blank. 

 There is conjugate deviation 

 of the head and eyes to the 

 same side as the lesion in 

 other words, the animal turns 

 its head and eyes to the right. 

 Destruction of this region on 

 both sides causes complete 

 blindness. When the same 

 region is stimulated, the eyes 

 and head are turned to the 

 left that is, there is conju- 



RF 



LO 



Fig. 394. Diagram of Relations of Occipital 

 Cortex to the Retinae. RO, LO, right and 

 left occipital cortex; RE, LE, right and left 

 retina; C, optic chiasma; RF, LF, right and 

 left visual fields. The continuous lines 

 passing back from the retinae to the occi- 

 pital cortex represent the crossed, the 

 broken lines the uncrossed, fibres of the 

 optic nerves and tracts. For the sake of 

 simplicity the intermediate stations on the 

 visual path in the anterior corpora quadri- 

 gemina, lateral geniculate bodies, and pul- 

 vinar are not represented in the diagram. 

 For these connections, see Fig. 370, p. 923 



gate deviation to the opposite 

 side. In the higher monkeys the eye movements can be elicited only 

 from the extreme posterior apex of the occipital lobe and from its 

 calcarine region, and then not easily. The movements differ from 

 those produced by stimulation of the area for eye movements in the 

 frontal lobe. They are not so certain, their latent period is longer, 

 and a stronger stimulus is required to evoke them. It cannot be 



