3 5^ Joiiriia! of Cmiif'drali'c'r Xeiiroloc!;}' "»</ [\';y(iio!(\<^\'. 



IV. Coinf^ariioii Br/iorr/i tlic Nervous System of the Cmiiiea 

 Pig and that of the White Rat. 



At the end of the stud}' upon the ps)-chical processes of 

 the guinea pig a comp.u'ison is made between the psychical and 

 phx'sical development of the guinea jMg and the white rat. It 

 was found that the white rat is born extremely helpless and im- 

 developed, whereas the guinea pig is independent and well de- 

 veloped. On the ps)'chical side the guinea pig has reached the 

 limit of his possibilities by the third day, while the white rat 

 reaches ps)xhical maturity from twenty-three to twenty-seven 

 days. It was suggested that the contrast between the nervous 

 systems of the guinea pig and of the white rat might be^analo 

 gous to the contrast in their physical and psychical develop- 

 ment. 



We have seen in detail what is the condition of the guinea 

 pig's nervous system at birth, and what changes occur during 

 progress to maturity. 



A description of the same cycle for the white rat will be 

 found in Animal Education, pp. 87-111. Summarizing these 

 results we find tiiat in the white rat there are no medullated 

 fibers present at birth ; that in the spinal cord certain tracts 

 (pyramidal tracts, fasciculus gracilis, etc.) medullate slowly, and 

 that even in the adult the pyramidal tracts do not stain com- 

 pletely ; that development in the cerebellum is more rapid than 

 in the cerebrum ; that medullation in many regions of the cere- 

 brum is very slow up to the twenty-fourth day, alter which the 

 fibers rapidly mature. 



Now, if we compare the sections of the guinea pig's spinal 

 cord with sections of the white rat's spinal cord we see that the 

 guinea pig at birth has reached the same stage of de\X'lopment 

 attained by the white rat at twenty-four days. Similarity be- 

 tween the new-born guinea pig's psychical processes and those 

 of the twenty-four day white rat is correlated with surprising 

 accuracy with the similarity in their nervous systems. The 

 similarity is present in a less marked degree between the cerebral 



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