68 



KINETIC HORMONES — I 



2 3 4 5 6 7 8 9 10 II 



Time, min 



12 13 14 15 16 17 18 



Fig. 3-7. Effect of oxytocin on milk "let-down" in the nursing 

 bitch, Canis. The time in minutes is given as abscissae and the 

 amount of the milk yield (as indicated by the increase in weight 

 of the pups) as ordinates. In the normal case, the pups get all the 

 milk that the mammary glands can yield in about 8 min, there 

 being an initial latent period before milk becomes available to 

 them and a falling off as the gland becomes exhausted. In the 

 right-hand curve, starting again at time 0, the pups were put to the 

 anaesthetized mother and failed to obtain more than 30 per cent 

 of the milk, after a very slow start. After 7 min an injection of 

 0.5 ml Pituitrin containing oxytocin enabled the pups to obtain 

 almost all the rest of the milk, presumably by causing contraction 

 of the myoepithelial cells round the alveoli. The injection of 

 oxytocin has no effect upon the amount of milk secreted, as can 

 be seen by the lack of effect of similar injections made when the 

 curves had already exceeded 90 g (from Gaines, 1915). 



In the vertebrates, most types of visceral muscle which are 

 innervated by the sympathetic fibres of the autonomic nervous 

 system are thus seen to react also to secretion from the adrenal 

 medulla; but the nerves of mammals are now known to secrete 

 mostly noradrenaline at their end-plates, whereas the greater part 

 of the gland secretion (derived from neurosecretory cells which 

 originated in sympathetic ganglia) is adrenaline itself, although it 

 is admixed with larger amounts of noradrenaline in the lower 

 vertebrates, e.g. in the secretion of the suprarenal bodies of the 

 elasmobranchs. 



