POSTNATAL GROWTH IN THE ALBINO RAT 41 



IJj.. General considerations 



We may now review briefly certain phases of growth and varia- 

 bihty of the viscera. First should be noted the growth of the 

 viscera, taking the group as a whole (including brain), in com- 

 parison with the remainder of the body. As found by Jackson 

 and Lowrey ('12), the visceral group of the rat at birth forms 

 about 18 per cent of the body weight. It increases in relative 

 weight to an average of about 19.2 per cent at seven days, 

 and to a maximum of about 21.3 per cent of the body weight 

 at three weeks. At six weeks, it has decreased slightly, to 20.4 

 per cent, but continues to decrease in relative weight to about 

 16 per cent at ten weeks, 14.8 per cent at five months, and 13.3 

 per cent at one year. 



Data for comparison of the relative size in other animals are 

 cited by Jackson and Lowrey ('12), who point out that in gen- 

 eral the smaller mammals have a relatively larger visceral appa- 

 ratus, probably correlated with a more intense metabolism. The 

 rat occupies a somewhat intermediate position, the relative weight 

 of the viscera being less than that of most of the smaller mammals, 

 but greater than that of the larger mammals. 



Since the visceral group forms a comparatively small part of 

 the entire body, its relative size will evidently be influenced greatly 

 by the rate of growth of the other parts of the body. . Jackson 

 and Lowrey ('12) have shown that in the rat the relative increase 

 in the weight of the viscera during the first week is accompanied 

 by a similar slight increase in the skeleton and a very marked 

 increase in the relative weight of the integument. The relative 

 increase in these three systems is apparently balanced by a small 

 decrease in the relative weight of the musculature, and by a 

 remarkable decrease in the 'remainder,' due chiefly to a disap- 

 pearance of excess body liquids. The slight further increase in 

 the relative weight of the viscera at three weeks is accompanied 

 by an increase in the musculature, and is balanced by decreases 

 in the skin, skeleton and remainder. Thereafter the slow gradual 

 decrease in the relative weight of the visceral group is accom- 

 panied by a similar decrease in the skin and skeleton. This is 



