452 



THE INTERNAL STRUCTURE 



4. The Principal Commissures of the Brain. 



The connecting or, as Meynert terms them, the ' associa- 

 tion system ' of fibres of the Brain belong to three principal 

 categories, each of which will be now briefly described. 



FIG. 162. Longitudinal Section through the centre of the Brain, showing the 

 inner face of Left Cerebral Hemisphere. (Sappey, after Hirschfeld.) 1, Spinal 

 Cord; 2, Pons Varolii ; 3, Cerebral Peduncle; 4, 'Arbor Vitas' of cut surface of 

 Middle Lobe of Cerebellum ; 5, Sylvian aqueduct ; 6, Valve of Yieussens ; 7, Corpora 

 quadrigemina ; 8, Pineal body ; 9, its inferior peduncle ; 10, its superior peduncle ; 

 II, middle portion of the great Cerebral Cleft; 12, upper face of the Thalamus ; 

 13, its internal face, forming one of the walls of the middle or third ventricle ; 

 13', Grey or Middle Commissure ; 14, Choroid plexus ; 15. Pituitary pedicle ; 

 10, Pituitary body ; 17, Tuber cinereum ; 18, Marnmillary body ; 19, interpeduncular 

 perforated lamella; 20, third or common Oculo-motor nerve; 21, Optic Nerve; 

 22, Anterior Commissure ; 23, Foramen of Monro ; 24, section of the Cerebral 

 trigone ; 25, Septum lucidum ; 26, Corpus Callosum ; 27, its posterior extremity or 

 'bourrelet ' ; 28, its anterior extremity or 'genu ' ; 29, 30, Gyrus fornicatus, or Con- 

 volution of the Corpus Callosum; 31, Anterior part of Marginal Convolution 

 32, Calloso-marginal sulcus ; 33, Occipital Convolutions ; 34, ' internal perpendicular 

 fissure ' separating Occipital from Parietal Lobe. 



These fibres are of great importance, and so numerous, 



that, Broadbent says,* " the radiating fibres must bear a 



* " Journ. of Ment. Science," Ap. 1870, p. 9. 



