186 PROBLEMS OF RELATIVE GROWTH 



ance of function, as shown by the fact that young mammals 

 can live fairly healthily when kept at constant weight or even, 

 for a time, decreasing weight, and that planarians, etc., can 

 keep healthy though enormously reduced in bulk by starva- 

 tion. In organs where, to use Hammett's phrase, the work- 

 growth ratio is high, as in glands, heart, etc., since a decrease 

 of metabolism falls more heavily on the growth-function than 

 the work-function, therefore the growth-function will be more 

 seriously impaired than in organs such as C.N.S. or skeleton, 

 where the work-growth ratio is low. And the first point, the 

 greater influence of thyroid-deficiency on growth by cell-size 

 increase, is perhaps due to a specific action of thyroid, though 

 it too could be interpreted in terms of a greater effect of thyroid- 

 depression on growth than on function or work, the cell which 

 is growing in bulk being usually already differentiated for its 

 definitive function. 



Hammett also considers a ' special group ' of organs. The 

 thymus is noted for its sensitivity to all unfavourable con- 

 ditions ; and in general is the most adversely affected of all 

 organs by thyroid removal. The testes are resistant because 

 continuously producing new cells, while the ovaries, after 

 puberty at least, are highly affected. The pituitary is known 

 to show compensatory hypertrophy on thyroid removal, and in 

 consequence increases its growth, notably in males (Fig. 83). 



There is also a specific sex difference, females showing a 

 greater effect of thyroidectomy as regards body-weight and 

 almost all organs. And finally, puberty accentuates the 

 sensitivity of all the sensitive organs. 



The net result of thyroidectomy is the production of an 

 absolutely smaller 150-day animal, but one which is abnormally 

 slender, and has relatively larger eyes, testes and seminal 

 vesicles, nervous and skeletal systems than a control of the 

 same age, but relatively smaller viscera and glands ; further, 

 the normal sexual size-difference is increased. When compared 

 with animals of the same body- weight, the resistant groups of 

 organs are, of course, absolutely as well as relatively larger 

 (Hammett, 1929A). 



Hammett further states that there is no correlation between 

 the degree of sensitivity to thyroidectomy and the normal 

 growth-rate (growth-coefficient) of organs. However, he has 

 for the most part compared qualitatively different organs, in 

 which the work-growth ratio, the cell-multiplication : cell-size- 

 increase ratio, and the ratio of labile to stabile materials may 



