LECTURES IN BIOLOGICAL SCIENCES 



istics. The brain surface in man is highly convoluted. A large 

 sheet of cerebral cortex in man is packed into a cerebral box, 

 the human cranium. II spread out flat the human cerebral 

 cortex would be a square of tissue about 2 feet at the edge. 

 Mechanically it is the problem of putting a napkin 24 x 24 

 inches into an 8-inch bowl. It has to be folded up to get in. 

 But the cortex grows up within the skull. As it increases in 

 volume, it takes on folds to fit within the endocranium. Some 

 of the folds are mechanical, to go around bends in the base or 

 other parts of the growing container and brain. Other folds 

 reflect internal differences in the cellular make-up of the cortex 

 itself. Before we take up certain evolutionary changes in homi- 

 nid brains, let me remind you of the general surface anatomy 

 of that organ. 



The cortical surface of the brain is folded into convolutions 

 separated from each other by depressions. The longer and 

 deeper depressions are called fissures or sulci. The longest and 

 deepest fissure (the lateral fissure of Sylvius) begins on the part 

 of the brain lying behind the eye and beneath the temple and 

 passes backwards and upwards to and above the position of the 

 top of the ear. The frontal and parietal lobes of the brain are 

 situated above, and the temporal lobe below the lateral fissure. 

 A second large fissure (the central fissure of Rolando) crosses 

 the surface of the cerebral hemisphere from above downwards 

 and separates the frontal from the parietal lobe. The central 

 fissure runs from a position at the top, close to the vertex of the 

 head, obliquely forward in the direction of a point lying be- 

 neath the cheek arch. 



This brings us to the question of how much we can learn 

 about the brains of ancient primates from skull parts preserved 

 as fossils. We know about over-all size and general shape with 

 considerable accuracy. The patterns of the meningeal blood 

 vessels are often beautifully reflected on the endocranial walls, 

 but we have yet to learn the significance for behavior, if any, 

 of variation in these patterns. Since the brain makes a partial 

 record of its convolutional complexity on the skull which con- 

 tains it, we can have a rough idea of the surface anatomy of the 

 brain from the skull alone. A check on the reliability of this 

 procedure is available by taking endocranial casts of modern 

 primates whose brains are available for study. Unfortunately 



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