WEIGHTS OF ORGANS IN UNDERFED YOUNG RATS 



129 



BRAIN 



The brain (table 10) in eleven controls at three weeks of age 

 averaged 1.282 grams, or 5.31 per cent of the (net) body-weight. 

 This corresponds fairly closely with Donaldson's ('08) figure for 

 the normal rat of corresponding weight. In the rats held at 

 constant body-weight from the age of three to six and eight 

 weeks, there appears (table 10) to be a relative increase in the 

 brain, especially at eight weeks, where it forms 6.62 per cent of 

 the body. In reality, however, this relative increase is only 

 apparent, and due to the fact that these animals began the ex- 

 periment at a lower body-weight, corresponding to which the 

 brain is relatively heavier. The (net) body-weights of the two 

 rats at eight weeks were respectively 18.1 grams and 17.8 grams. 

 According to Jackson ('13, p. 22), the brain normally reaches its 

 maximum relative weight of about 6.7 per cent of the body when 

 the body- weight is about 15 grams. Thus the final brain-weight 

 of the rats held at constant body-weight from the age of three to 

 eight weeks of age is almost exactly that to be expected if the 

 brain- weight has remained constant. 



TABLE 10 



The brain; average absolute weight, average -percentage of net body-weight and range 



indicated 



DESCRIPTION OF RATS 



ABSOLUTE WEIGHT 

 (AND RANGE): GRAM3 



relative weight 

 (and rangej : 



PER CENT 



Normal at 3 weeks (Donaldson '08, table 1) 



Controls at 3 weeks 



Body-weight constant 3 weeks (age of 3 to 6 weeks) 

 Body-weight constant 5 weeks (age of 3 to 8 weeks) 

 Body-weight constant 7 weeks (age of 3 to 10 weeks) 



Normal at 6 weeks (Donaldson '08) 



Control at 6 weeks 



Body-weight constant 26 weeks (age of 6 to 32 weeks) 



Normal at 10 weeks (Donaldson '08) 



Controls at 10 weekst 



Body-weight constant 25 weeks (age of 10 to 35 weeks) 

 Controls at 32 and 35 weeks 



1.285 



1.282 (1.187-1.364) 



1.195 (1.035-1.297) 



1.183 (1.180-1.186) 



1.267 (1.136-1.379) 



1.441 



1.373 



1.478 (1.459-1.497) 



1.559 



1.579 (1.512-1.636) 



1.646 (1.603-1.723) 



1.777 (1,657-1.890) 



5.14 



5.31 (4.14-6.20) 



5.44 (4.50-6.33) 



6.62 (6.60-6.63) 



5.30 (4 38-6.20) 



3.20 



3.23 



3.14 (2.96-3.32) 



2.08 



1.21 (0.96-1.44) 



2.12 (2.02-2.18) 



0.97 (0.78-1.24) 



* Gross body- weight. 



t Body-weight ot controls at 10 weeks too high for comparison. 



