IOQ4 REPRODUCTION 



abdominal muscles, and, if the woman is not anaesthetized, also by 

 voluntary contractions of these and of other muscles, which can 

 increase the intra-abdominal pressure. The uterine contractions 

 can be initiated and modified by impulses coming from the central 

 nervous system by way of the extrinsic nerves of the organ. It is 

 known, e.g., that the gravid uterus can be excited to contraction by 

 the stimulation of various sensory nerves. Powerful mental impres- 

 sions, such as fright, may bring on premature labour. Conversely, 

 sudden cessation of labour pains during parturition is not uncom- 

 monly observed to be produced by emotional disturbances for 

 instance, the entrance of a stranger into the room. Yet the con- 

 tractions of the uterus are not essentially dependent upon extrinsic 

 impulses. For not only do rhythmical contractions occur, but the 

 whole process of parturition has been seen to take place in a uterus 

 whose nerves have all been cut. Even the excised uterus may be 

 kept alive for as long as forty-eight hours, and may go on executing 

 periodical contractions when its bloodvessels are perfused with such 

 an artificial fluid as Locke's solution, or, indeed, when it is simply 

 immersed in the oxygenated solution (Kurdinowski) (Practical 

 Exercises, p. 1101). 



It is a question of great interest how the uterine contractions are 

 started so abruptly at full term after so long a period of quiescence. 

 It can hardly be that the increasing mechanical distension of the 

 uterus, tolerated for so many months, should suddenly, in an hour, 

 become intolerable. For if the foetus dies before full term it is 

 expelled without reference to the bulk which the uterus has reached. 

 It is more likely that some chemical change associated with the 

 completion of intra-uterine development, a change which leads, 

 perhaps, to the production of some specific substance in the placenta 

 or the feet us, is the determining event. The placenta is a structure 

 whose function is strictly limited to the term of intra-uterine develop- 

 ment. The foetus is to live on, and so is the mother. May it not 

 be that the placenta or essential elements in it are timed to die, or 

 to begin to die, at full term, and that in their death or degeneration 

 the substance or substances are produced which start, and later 

 sustain, the uterine contractions ? And may not the contractions of 

 the uterus, by exciting its afferent nerves, or through the pressure 

 of the foetus the afferent nerves of the vagina, in turn evoke the 

 associated reflex contractions of the abdominal walls ? These are 

 questions which have been asked, but not as yet satisfactorily 

 answered. It has also been suggested that a hormone formed in the 

 mammary gland at full term stimulates the uterus and thus brings 

 on labour. 



At birth, great changes take place in the foetal circulation, and these 

 are intimately connected with the commencement of the respiratory 

 activity of the lungs. The causes of the first respiration are: (i) The 



