572 NERVOUS SYSTEM 



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3. Association fibres connect cBferent parts of the cortex on the 

 same side. These constitute the gr^Mfc part of the white substance. 



4. Centripetal fibres, coming from mRentripetal tracts of the cord, 

 go to the molecular layer of the cortex. 



In addition to the above, there are fibres that connect the cerebrum 

 with the cerebellum, which will be described later. 



Cerebral Convolutions. The cerebrum presents a great longitudinal 

 median fissure by which it is partially divided into two lateral halves, 

 and three great fissures the fissure of Sylvius, the fissure of Rolando 

 and the parieto-occipital fissure. The lobes of the cerebrum are (i) the 

 frontal lobe, lying in front of and above the fissure of Sylvius and in 

 front of the fissure of Rolando, (2) the parietal lobe, behind the frontal 

 lobe and in front of and above the occipital lobe, (3) the occipital lobe, 

 and (4) the temporo-sphenoidal lobe. The parietal lobe is bounded in 

 front by the fissure of Rolando and below by the fissure of Sylvius 

 and the parieto-occipital fissure (shown in Fig. 134). The occipital lobe 

 lies below the parieto-occipital fissure. The temporo-sphenoidal lobe is 

 situated below the fissure of Sylvius and in front of the occipital lobe. 



While the convolutions are not exactly the same in all human brains,, 

 or even in both sides of the brain, their arrangement and relations may 

 be described in a general way with sufficient accuracy to enable one to 

 recognize easily the most important physiological points in the descrip- 

 tive anatomy of the cerebral surface. 



The first frontal convolution is bounded internally by the great longi- 

 tudinal fissure and externally by a shallow fissure nearly parallel to the 

 longitudinal fissure. The second frontal convolution lies next the first 

 frontal convolution, and is bounded externally by two shallow fissures 

 lying in front of the fissure of Sylvius. The third frontal convolution 

 curves around the short branch of the fissure of Sylvius. On either 

 side of the fissure of Rolando are the ascending frontal convolution 

 and the ascending parietal convolution. Curving around the posterior 

 extremity of the fissure of Sylvius, is the supramarginal convolution, 

 which is continuous with the first temporal convolution, the latter lying 

 behind and parallel to the fissure of Sylvius. Internal to the posterior 

 portion of the intraparietal sulcus is the angular convolution, which is 

 continuous with the second temporal convolution. At the inferior 

 border of the temporo-sphenoidal lobe, below the first and second 

 temporal convolutions, is the third temporal convolution. The superior 

 parietal convolution lies by the side of the median fissure and is the 

 posterior continuation of the first frontal convolution. The situation 

 of the occipital convolutions is indicated in Fig. 139. In addition 

 to these convolutions on the general surface of the cerebrum, there 



