1. Adams JQ, Alexander AM (1958) Alterations in cardiovascular physiology during labour. Obstet Gynaecol 12:542
2. Akil H, Watson SJ, Young E, Lewis ME, Khachaturian H, Walker JM (1984) Endogenous opioids: biology and function. Ann Rev Neurosci 7:223
3. Basbaum AI, Fields HL (1978) Endogenous pain control mechanisms: review and hypothesis. Ann Neurol 4:451
4. Bonica JJ (1967) Principles and practice of obstetric analgesia. Davis, Philadelphia
5. Bonica JJ (1980) Labour pain: mechanisms and pathways. In: Marx GF, Bassell GM (eds) Obstetric Analgesia and Anesthesia. Elsevier/North Holland Biomedical Press, New York, p 173