64 KINETIC HORMONES — I 



to be a neuro-secretory product. It is absent in the horse, EquuSy 

 which has no gall-bladder. 



Gastrin, secreted by the stomach, can induce the contraction of 

 stomach muscles, forcing food into the duodenum as the gastric 

 phase of digestion is completed; but this action of gastrin 

 follows, and is subsidiary to, that of stimulating the acid-secreting 

 gland cells of the stomach (§4.111). The contraction is said to be 

 inhibited by enterogastrone ; but this action is not purely hor- 

 monal, since it is more effective if the vagus nerves are intact 

 (Grossman, 1950). The nervous inhibition can be stimulated by 

 acid in the duodenum. 



3.113 Muscles of the genital ducts 



No cases seem to have been recorded so far in which the muscles 

 of any part of the genital ducts of an invertebrate are controlled 

 by hormones. 



Mammalia. Isolated uterine muscle of many mammals, such as 

 the rat, Rattus, reacts to oxytocin from the neurohypophysis by 

 strong rhythmic contraction (Fig. 3-4). At the end of pregnancy 

 these contractions force the embryo out of the uterus. The fact 

 that this does not normally occur until the embryo is fully devel- 

 oped seems to be due, not to a lack of oxytocin during earlier stages 

 of pregnancy but to changes in the level of sensitivity of the uterus. 

 In the rabbit, Oryctolagus, the relative insensitivity of the uterus 

 to oxytocin during the 30 days of pregnancy, as compared with 

 variability at other times, is due to the abundant presence 

 of progesterone. As this decreases and oestrogen increases in 

 the circulation towards the end of pregnancy, the uterus 

 becomes increasingly sensitive to the oxytocin until finally 

 it reacts sufficiently strongly to bring about parturition 



Earlier experiments, in which it was found that rats from which 

 the hypophysis had been removed were still able to produce their 

 litters successfully, have now been explained on the grounds that 

 the neurohypophysis is only a storage organ for hormones secreted 

 in the hypothalamus of the brain (§ 2.111), and the experimental 

 technique therefore failed to remove the source of the oxytocin. 

 In those cases where the source was also destroyed by hypothala- 



