CEREBEUM 193 



causing its natural response, it is justifiable to conclude that 

 that part of the brain is connected with reception. 



2nd. In man, the chief difficulty of obtaining information 

 is in finding cases where only a limited part of the brain is 

 affected. But such cases have been observed. Tumours of 

 the inner aspect of an occipital lobe, for instance, have been 

 found to be associated with loss of visual sensations without 

 loss of muscular power, and thus the conclusion has been 

 drawn that this part of the occipital lobe is the receiving 

 mechanism for stimuli from the eyes. 



3rd. When it has been found possible to assign a definite 

 function to any area of the cortex, the extent and limits of 

 the area may be determined by the extent and distribution 

 of the particular character of the arrangement and structure 

 of the nerve cells. 



4th. Flechsig has found that bands of fibres going to certain 

 parts of cortex get their medullary sheaths earlier than others, 

 and that the fibres to each part of the cortex become medullated 

 at a definite date. The areas, the fibres of which first get their 

 sheath, he calls the primary projection areas, and they correspond 

 very closely with the receiving areas determined by other 

 methods (fig. 100 A and B). 



Visual Centre (fig. 97). The way in which the fibres, coming 

 from the two retinae, are connected with each thalamus opticus 

 and occipital lobe has been already considered, and it has been 

 shown that the optic tract passes into the geniculate bodies 

 on the posterior aspect of the thalamus, and that a strong 

 band of fibres, the optic radiation, extends from these back- 

 wards to the occipital lobes (fig. 67, p. 138). In man an ex- 

 tensive lesion of one say the right occipital lobe, especially if 

 on the inner aspect round the calcaririe fissure, is accompanied 

 by no loss of muscular power but by blindness for all objects 

 in the opposite side of the field of vision i.e. the right side 

 of each retina is blind. The central spot of neither eye is 

 completely blinded because the fibres from the macula lutea 

 only partially decussate at the chiasma. 



Stimulation in this region causes movement of the eyes to the 

 opposite side, as if some object were perceived there. 



Histologically this area is characterised by a well-defined 



13 



