THE NERVOUS SYSTEM 



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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 

 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. 



Following this brief outline of the main advances in the nervous system 

 as represented in living chordates, it seems desirable at the risk of some 

 repetition to present separately and in greater detail the evolution of 

 the three major divisions of the nervous system: i. The evolution of the 

 Central Nervous System (Brain and Spinal Cord). 2. The Evolution 

 of the peripheral nerves (cranial and spinal). 3. The Evolution of the 

 Autonomic or Sympathetic System. 



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 regions. 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 developed in apes. The brains of such fossil types as the Java 

 and Peking man are transitional between those of modern man and apes. 



Nor is the gap between the brain size of mammals and reptiles formid- 

 able. In a dorsal view, all five divisions of the primitive brain are visible 

 alike in monotremes and alligators. The overgrowth of the hemispheres, 

 begun in reptiles, reaches its climax in man, whose domination in the 

 animal world may be ascribed to the enlargement of his conscious control 

 centers in the hemispheres. 



The cortical enlargement in mammals, however, involves more than 

 an increase of gray matter. Correlated with the multiplication of cells 

 is an increase in the number of nervous interconnexions. Association 

 fiber tracts connect all parts of the enlarging brain so that all regions, 

 however remote from one another, are interconnected. The brain 

 produced by these evolutionary changes is an organized and integrated 

 whole, no part of which appears to function independently of the rest. 



In primates a marked retrogression of the olfactory lobes accompanies 

 the enlargement of the hemispheres. The olfactory centers in the hip- 

 pocampus persist, but other regions of the cortex enlarge dispropor- 



