/Dorsal root 



IN AND OUT NERVE PATHS 



If the dorsal root of an intervertebral nerve is cut or broken, stimulating the end away 

 from the spinal cord produces no effects; but stimulating the end connected with the 

 cord arouses the same sensations as stimulating the corresponding sensory endings. 

 That is, this branch transmits impulses only toward the brain. If the ventral branch 

 of the nerve is cut or broken, stimulating the end near the cord produces no results; 

 but stimulating the portion away from the cord arouses muscular contractions or 

 glandular secretions, or both. That is, this branch transmits impulses only outward 

 from the center 



that of any of these animals. Thus the ratio of brain-weight to body-weight 

 is 1:40-42 in man; 1:500 in the elephant; and 1:12,000 in the humpbacked 

 whale. On the other hand, some of the smallest mammals have relatively 

 larger brains: the ratio of brain- weight to body-weight is 1:22-26 in some of 

 the marmosets and some fancy breeds of mice, and even more in some of the 

 spider monkeys — about 1:17-20 (see table, p. 297). 



The cortex, or "rind", of the cerebrum consists of nerve cells. In mam- 

 mals this gray layer is very much wrinkled, so that there is relatively more 

 surface than in lower vertebrates. The primates have more complex brains 

 than other mammals. The cortex of the primates has five distinct layers of 

 cells, as against three in other mammals. This fact is apparently related to 

 the greater capacity of primates to learn ; and in the case of man this means 

 also the capacity to imagine, to form and to remember abstract ideas, to 



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