362 



CHORDATE ANATOMY 



a surface eighteen inches square. The number of neurons runs into the 

 biUions, and five layers of cells may be distinguished. The increase in the 

 mass of the cerebral hemispheres as we pass from lower to higher mammals 

 is the result, not of multiplication of layers of neurons in the cerebral 

 cortex, but of folding of the cortex. 



A notable development is that of the anterior pallial commissure, 

 which enlarges enormously to form the corpus callosum, interconnecting 

 the two hemispheres. The olfactory lobe degenerates and is covered 

 by the hemispheres. The corpora striata elongate caudally and rest 



MASSA INTERMEDIA THALAMI 

 SULCUS CINGUL 

 GYRUS CINGULI 



SUPERIOR 

 FRONTAL GYRUS 



FORNIX 



frontal pole ' 

 anterior commissure- 

 terminal lamina 



optic nerve- 

 optic chiasma'' 

 infunoibulum 

 hypophysis' 

 mammillary body 

 oculomotor nerve 



THALAMUS 



'CENTTRAL SULCUS 



TELA CHORIOIDEA C3RD VEKTT.) 

 CORPUS CALLOSUM 



, POSTERIOR COMMISSURE 

 , PINEAL BODY 



SUBPARIETAL SULCUS 



' LAMINA QUADRIGEMINA 



OCCIPITAL LOBE 



CALCARINA FISSURE 



LINGUAL GYRUS 

 OCCIPITAL POLE 



PONS' 

 FOURTH VENTRICLE' 



MEDULLA ' 

 CENTRAL CANAL ' 



^'CEREBELLUM 

 TELA CHORIOIDEA t4TH VENT.) 



\^-SPINAL CORD 



Fig. 321. — The human brain in median section and aspect. (Redrawn after Sobotta.) 

 Besides the enormous enlargement of the cerebral hemispheres and cerebellum in the 

 human brain, the great size of the corpus callosum and the pons Varolii is noteworthy. 



upon the thalami. The epiphysis forms a gland, the pineal. Cerebellum 

 and pons enlarge. The pons is a bridge of nerve fibers, present only in 

 birds and mammals, which extends around the brain-stem ventral to 

 the cerebellum, and which connects the two halves of the cerebellum. 



EVOLUTION OF THE BRAIN 



Comparison of vertebrate brains from cyclostomes to man reveals a 

 gradual and progressive change such as would be expected if the higher 

 forms have evolved from the lower. The cerebral hemispheres are the 

 least conservative region. Although the hemispheres are enormously 

 enlarged in man, the differences between man and apes are quantitative 

 rather than qualitative. Even the speech center in the frontal lobes, 

 which is said to be peculiar to man, is but an enlargement of regions already 



