420 HUMAN PHYSIOLOGY 



quiescent for a short time, and then contractions supervene, 

 and the placenta and lining of the uterus are expelled as 

 the after-birth. These uterine contractions are presided over 

 by a nerve-centre in the lumbar enlargement of the cord, and 

 in all probability the nervi erigentes play an important part 

 in their production. 



