CHEMICAL REGULATION OF METABOLISM 381 



one another, for while the variability of the weight of newborn infants 

 is 14 per cent., that of the length of the period of gestation is only 4 

 per cent. There can be little influence exerted by the size of the fetus 

 upon the length of gestation, therefore, for otherwise the variability 

 of the period of gestation would be nearly as great as the variability of 

 the size of the infants delivered. It is evident that heavy infants are 

 carried in utero for a longer period and light infants for a snorter period 

 than w r ould correspond to their relative development. 



We must therefore look to maternal rather than to fetal events for 

 the source of the determination of the period of gestation. Now the 

 investigation of the physiological condition of the mother yields indica- 

 tions of two factors which, as the term of pregnancy approaches, must 

 enhance the muscular irritability of the uterus. The first is the hyper- 

 trophy of the Pituitary Gland, to which reference has been made above. 

 The aqueous extract of the posterior lobe exerts a very marked effect 

 upon the excised uteri of animals, inducing powerful contractions, 

 especially in the pregnant uterus. The active constituent is related to 

 but not identical with /3-Iminazolyl Ethylamine. We may assume with 

 probability that the hypertrophy of the gland which accompanies 

 pregnancy may result in the presence of this substance in increasing 

 amounts in the blood-stream until, finally, the hyperirritability of the 

 uterus, with the assistance of the second active substance about to be 

 noted, reaches a stage culminating in contractions which expel the fetus. 



The second factor which operates in the direction of promoting 

 contraction of the uterus, is the presence of a substance io the Colostrum 

 or first secretion of milk, which causes contractions of the pregnant 

 uterus. In fact abortion has been brought about in pregnant cattle 

 before the normal period of delivery, by injections of colostrum from a 

 normal cow. Colostrum differs in many respects from the milk which 

 is subsequently secreted. This will be clear from the following analyses 

 of cows' milk, by Konig. 



Per 1000. Water. Solids. Casein. Other Fats. Sugar. Salts. 



protein. 



Colostrum . . 746.7 253.3 40.4 136.0 35.9 26.7 15.6 



Milk .... 871.7 128.3 30.2 5.3 36.9 48.8 7.1 



It has been recognized from a remote period that colostrum has a 

 cathartic action upon the infant, so that the substance inducing 

 uterine contractions may possibly be a general muscular stimulant. 

 Its chemical nature is, however, unknown. 



THE CHEMICAL REGULATION OF METABOLISM. 



The activities of our various tissues are so closely interwoven with 

 one another, and the various organs of the body are so intimately 

 dependent upon one another for the raw materials which they elaborate 

 into finished products, or the disposal of waste products which might 

 otherwise be deleterious to the well-being of the whole bodily economy, 



