748 



A MANUAL OF PHYSIOLOGY 



of lesions in the motor area of the cortex, and operative interference 

 for their cure, the exact position of the fissure of Rolando becomes 

 important ; and Thane has given the following simple method for 

 fixing it : The point midway between the root of the nose and the 

 occipital protuberance is fixed by measuring the distance with a 

 tape. The upper end of the fissure of Rolando lies half an inch 

 behind this middle point. The fissure makes an angle of 67 with 



the longitudinal fissure 

 (Fig. 269). 



Sensory Areas 

 Visual Centres. In 

 the occipital lobe an 

 area of considerable 

 extent has been 

 found, destruction of 

 which causes hemi- 

 anopia, i.e., loss of 

 vision in the corre- 

 sponding halves of 

 the retinae. Thus, if 

 the right occipital 

 cortex is destroyed, 

 the right halves of 

 the two retinae are 

 paralyzed, 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 completeblindness. When the 

 same region is stimulated, the eyes and head are turned to the 

 left that is, there is conjugate deviation to the opposite side. 

 The movements differ from those produced by stimulation of 

 the Rolandic area. They are not so certain, their latent period 

 is longer, and they are considered to be not direct, but 

 reflex movements. It cannot be doubted that the occipital 



FIG. 271. DIAGRAM CF RELATIONS OF OCCI- 

 PITAL 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 retinas to the occipital 

 cortex represent the crossed, the broken lines the 

 uncrossed, fibres of the optic nerves and tracts. 



