184 PROBLEMS OF RELATIVE GROWTH 



as measured in the way described) increases with the age of 

 the animal. To take but a few examples, the eye, one of the 

 organs least affected by thyroidectomy, in males operated at 

 twenty-three days, by 150 days had made nearly 100 per cent, 

 of the growth of the eyes of the controls in the same period. 

 For those operated at 100 days, however, it made less than 

 80 per cent, of the controls' growth between 100 and 150 

 days. 



In regard to total body-weight, males operated at twenty- 

 three days showed about 60 per cent, of the increment of the 

 controls, while for those operated at 100 days, the percentage 

 was below 30 per cent. And as regards kidneys, males operated 

 at twenty-three days showed under 50 per cent, of the con- 

 trols' increment, those operated at sixty-five days showed hardly 

 any increase at all, while those operated at 100 days, had 

 before 150 days lost in absolute kidney-weight an imount 

 equivalent to some 50 per cent, of the increment made by 

 the controls in the same period. Thus the sensitivity of 

 growth to thyroid-deficiency increases with age. 



Secondly, most of the organs of the body fall into distinct 

 groups as regards differential sensitivity to thyroidectomy. 

 The eye-balls, central nervous system, length-growth both of 

 body and tail (doubtless determined by growth of the axial 

 skeleton), and both length- and weight-growth of humerus and 

 of femur are relatively resistant, being retarded less than the 

 weight of the body as a whole. On the contrary, the adrenals, 

 spleen, kidneys, liver, heart, submaxillary glands, pancreas 

 and to a slighter extent the lungs, make less increment than 

 general body-weight (Fig. 82). 



Hammett points out that these results are all consistent 

 with the idea that the influence of thyroidectomy is greater 

 on growth by increase of cell-size than on growth by cell- 

 multiplication ; greater on that fraction of the metabolism 

 concerned with function (work) — e.g. secretion, muscular 

 activity, etc. — than on that concerned with growth ; and 

 greater on labile than on stable chemical compounds. The 

 last point is a correlate of the obvious fact that in conditions 

 of subnormal nutrition (as in thyroidectomy) labile materials, 

 such as the contents of glandular tissues, are more readily 

 drawn upon than stable or inert substances, such as the salt- 

 deposits in the skeleton, the lipoids of the nervous system, 

 or the humours of the eye. The second is a correlate of the 

 fact that in malnutrition, growth gives way to the mainten- 



