666 



STRUCTURE OF THE CEREBRUM 



[CH. XLVI. 



internal parieto-occipital fissure, which is a continuation of the external 

 parieto-occipital fissure, passes downwards and forwards till it meets 

 the calcarine fissure ; these two enclose between them a wedge-shaped 



piece of brain called the cuneus or 

 cuneate lobule; the square piece above 

 it is called the precuneus or quadri- 

 lateral lobule. 



The only convolutions now left are 

 those which are placed on the surface 

 of the frontal lobe that rests on the 

 orbital plate of the frontal bone ; they 

 are shown in fig. 453, 2 2' 2" (p. 622), 

 and may be seen diagrammatically in 

 fig. 487, the end of the temporal lobe 

 being cut off to expose the convolutions 

 of the central lobe or Island of Eeil. 



Along the edge is the continuation 

 of the marginal convolution (M) ; next 

 comes the olfactory sulcus (o) in which the olfactory tract and bulb 

 lie ; then the triradiate orbital sulcus (o.s.) which divides the rest of 

 this surface into three convolutions. 



A.P.S 



FIG. 487. Orbital surface of frontal lobe. 



M, marginal convolution, 

 i O, olfactory sulcus. 

 -O.S., orbital sulcus. 

 i- I, island of Beil. 

 S.a., anterior limb of Sylvian fissure. 

 . S.p., posterior limb of Sylvian fissure. 

 A.P.S., anterior perforated spot. 



