GROWTH OF THE CORPUS CALLOSUM 



53 



It is of interest to note that at birth the callosal area is rela- 

 tively large compared with the area of total brain (table 2) and 

 furthermore that the thicknesses of the three parts (genu, 

 splenium, and truncus) are more similar at birth than they are 

 at later ages. Table 5 shows these latter relations. 



DISCUSSION 



I wish now to present some answers to the questions raised 

 at the beginning of this paper. 



1. If the callosum is taken as typical for the white matter of 

 the brain, then we may state the manner of its increase in the 

 following general terms: 



TABLE 6 



Showing the average values for the length and the thickness of parts of the callosum 



arranged in three age groups. Based on table 5 



AGE GROUP 



Birth to 22 days 

 25 to 80 days. . . . 

 90 to 378 days. . . 



1.0 



1.25 



1.40 



The first phase of growth is characterized by the addition of 

 new axones and the growth in diameter of the axones already 

 present in the callosum, but it is not till the end of this phase 

 that the myelin sheaths appear. 



In the second phase the addition of new fibers becomes rela- 

 tively less important — the greatest change is caused by the 

 rapid formation of the myelin sheaths, and in addition all the 

 fibers, myelinated and unmyelinated, increase in diameter. 



In the third phase growth in the diameter of the formed 

 fibers is the most important factor, and this apparently con- 

 tinues so long as the brain increases in weight. 



2. The second question touched the relation between the 

 growth of the callosum and the psychical development of the 

 rat. As we ascend in the mammalian series the relative impor- 

 tance of the callosum appears to increase. 



