446 REPRODUCTION AND DEVELOPMENT 



probably than any other circumstance, the rate being lowest in just those 

 localities where these considerations are most apt to be depressive and 

 prohibitive of conception. 



Parturition. This is the complex process by which the now complete 

 fetus and its accessories are forced into the world from its mother's 

 womb. This event, the especial field of obstetrics, we need discuss only 

 in its purely physiological aspects. 



Obstetricians divide the process of birth into three stages, partly for 

 convenience of description. The first stage extends from the first 

 effective contraction of the upper half of the uterus to the time when the 

 os uteri is forced open enough for the head of the child to pass through 

 it. The second stage lasts from then until the child is fully born. The 

 third stage comprises the expression of the membranes and placenta. 

 It is obvious that the first two stages are physiologically continuous with 

 each other. 



The causes of the beginning of delivery are doubtless several in 

 number, but are all included under the term ripeness; on the other 

 hand the occasion of the commencement of the event may be any one 

 of many. Here as elsewhere the conditions are numerous and com- 

 plex rather than single and simple. On the average, about 280 days 

 after the beginning of the last menstruation the fetus has reached ma- 

 turity, and then the metabolic balance between the oxygen and the carbon 

 dioxide in its tissues begins to lean toward asphyxia, and in consequence 

 the placenta begins to degenerate. Thromboses tend to form in the 

 placenta, and soon this organ begins to act like a foreign body and to 

 break away from the uterine wall. Perhaps the excess of carbon dioxide 

 passed into the maternal medulla oblongata actuates the parturition- 

 center as it always does the respiratory center. For several weeks or 

 even months prior to delivery the uterus shows painless rhythmic con- 

 tractions, these being either inherent in the smooth muscle-protoplasm 

 or directed by resident ganglia. Perhaps the placenta and fetus, begin- 

 ning now to act like foreign bodies, stimulate the mother's uterine center 

 in the spinal cord so that the rhythmic contractions increase in force up 

 to the effective degree. Such being perhaps the causes of the beginning 

 of labor, the slightest disturbance of the nervous apparatus, of the cir- 

 culation, or of the muscular (uterine) nutrition would set the reflex neuro- 

 muscular mechanism in action. Although the child may furnish some 

 of these exciting conditions, it can supply little or nothing of the motive 

 power for its own extrusion into the world. 



The first stage of actual labor begins by uterine contractions of in- 

 creasing severity. As is probably the case with most smooth-muscle, 

 the uterine walls have both a tonic mode of contraction and a rhythmic 

 mode. The former tends to lessen the diameter of the uterus and to 

 keep it small, while the latter sort of contraction is the slow, progressive 

 peristalsis characteristic of all the tubes and sacs. The peristalsis of the 

 uterus in labor affects only the upper part of the uterus down as far as 

 the "contraction-ring." Each "pain" lasts about a minute, but they recur 



