486 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



stuffs which have not yet come to comprise living matter, a process 

 which, in the case of the amino-acids at all events, forms a very large 

 proportion of the total metabolism of a normally nourished animal. 



If any of these several factors is decidedly altered in magnitude or 

 velocity a more or less marked effect upon bodily weight will ensue. 

 Thus if the rate of inflow be diminished beyond a certain point by an 

 insufficient dietary the nutrient level sinks and growth is retarded, or, 

 in the adult animal which has attained growth-equilibrium, the process 

 of growth may be reversed and loss of tissue occur. The extent of this 

 reversion is strictly limited in the more complex forms of metazoa by 

 the necessity of maintaining certain mechanical conditions : the integrity 

 of the skeleton, the functional ability of the digestive organs, the pulsa- 

 tion of the heart, the integrity of a closed vascular system, the coordi- 

 nating activities of the nervous system, and the continuance of respira- 

 tory movements. If any of these suffer in so complex an organization 

 as our own the whole must fail and death ensue. But in some less 

 complex forms, as in the fresh water worm Planaria, starvation actually 

 accomplishes Reversion of Growth until an embryonic stage of develop- 

 ment is regained (Child). 



If, on the other hand, the rate of inflow of nutrients be maintained 

 unaltered and the rate of outflow increased or diminished the rate of 

 accretion of tissue must obviously be affected to a proportionate degree. 

 In normal cases, since the rate of outflow or consumption of nutrients 

 for tissue-building purposes is determined by the relative magnitudes 

 of the specific velocity-constants of upbuilding and disintegration, the 

 rate of outflow will vary in different species and not improbably in the 

 two sexes of the same species, and to a certain extent in different 

 individuals. The environment, on the contrary, provided the inflow 

 of nutrients is maximal or at least sufficient, may be expected to play 

 little if any part in determining the rate of outflow. 



The rate of overflow is also conditioned primarily by internal regula- 

 tion, but we may observe the effects of its alteration in so far as the 

 nutrient-level of the amino-acids is concerned, by the pronounced effects 

 of hyper- or hypo-activity of the Thyroid upon the development of the 

 tissues. The administration of thyroid extract leads to a very decisive 

 increase in the rate of Deaminization of amino-acids, and in normal 

 adults who have attained growth-equilibrium, this, which involves a 

 fall of the nutrient-level, results in progressive loss of weight which 

 may, if it affects essential tissues, result in dangerous or even fatal 

 symptoms. The effects of hypo-activity are the opposite and the 

 excessive accretions of tissue not being uniformly distributed, aberra- 

 tions of growth occur which culminate in the condition of Myxedema. 

 In amphibians excision of the thyroid, as Gudernatsch has very 

 strikingly demonstrated, results in the arrest of Metamorphosis, 

 possibly because the degeneration of certain tissues which is a necessary 

 precedent of metamorphosis cannot occur. 



