XXX.] THE BRAIN. :>'2~> 



8. Dorsally again of this is the third or median 

 convolution having a longitudinal fissure in its 

 posterior part. 



9. Between the third convolution and the median 

 longitudinal fissure is the fourth or superior 

 convolution. 



10. Looking at the median surface of the brain ex- 

 posed by the cut made previously ( 3), observe 

 that the superior convolution stretches down to 

 the supra-callosal convolution (cp. 4). 



11. In the anterior part of the dorsal surface of 

 the superior convolution, note the deep crucial 

 fissure running transversely and a little for- 

 wards, follow it on the median surface of the 

 brain ; it is seen to run into the fissure that lies 

 above the supra-callosal convolution. 



12. The bend of the fourth convolution around the 

 crucial fissure is the sigmoid gyms, the part 

 in front of it being the anterior limb, the part 

 behind being the posterior limb. 



13. Anteriorly and posteriorly the four external 

 convolutions join. 



The above general plan of the convolutions is in 

 special cases modified to some extent by secondary 

 fissures, and by fissures being absent in part of their 

 normal course (thus the fissure between the 1st and 

 2nd convolution is not infrequently incomplete) so 

 that the typical arrangement given above may not 

 be obvious at first 



