THE DEVELOPMENT OF THE EMBRYO 1145 



ever, the suffering is very slight and labor is completed almost 

 without pain. 



These contractions possess a peristaltic character and may develop 

 a pressure of 30 pounds, the average being 17 pounds; in fact, in rare 

 instances pressures of from 50 to 100 pounds have been encountered. 

 It should not be forgotten, however, that the actual expulsion of the 

 fetus also brings into play the abdominal press which greatly 

 augments the force of the uterine musculature. In addition, the pa- 

 tient braces her body and contracts other muscles to steady herself. 

 The frequency of the heart and the arterial pressure increase during 

 the contractions, whereas the respiratory rate decreases. The process 

 of labor is usually divided into three stages, namely: 



(a) From the beginning of the first cramp-like pains to the completion of the 

 dilatation of the cervix ; leading to the rupture of a few local blood-vessels and the 

 discharge of the amniotic fluid. 



(6) From the rupture of the membranes to the complete delivery of the child. 



(c) The placenta separates from the uterine wall and is expelled together with a 

 small quantity of blood (500 c.c.). The uterus gradually recedes, forming a solid 

 tumor well below the umbilicus. 



The average duration of labor in primiparsB in 18 hours; 16 hours 

 of this period being consumed by the first, 1 hour and 45 minutes by 

 the second, and 15 minutes by the third stage. It is usually more pro- 

 longed in elderly women, but is much shorter in multiparse. Labour 

 is essentially a reflex process in which the uterine musculature plays 

 the principal part. The correctness of this deduction is proved by 

 the fact that even a uterus the nerves of which have been divided, is 

 capable of successfully expelling its contents. Consequently, we may 

 omit many of the theories which have been formulated to explain the 

 onset of labor and confine ourselves to those which hold that this 

 process is not dependent upon a stimulation of certain nerve centers, 

 but is instigated by a local stimulus either in the form of mechanical 

 impacts or in the form of a hormone contained in the blood stream. In 

 the first instance, the presumption would be that the steadily growing 

 fetus eventually produces a maximal distention of the uterus and 

 thereby incites a contraction of its musculature. But this view does 

 not coincide with the observation that large fetuses are often retained 

 for a much longer time than those of smaller size. Among the chemical 

 theories might be mentioned the one advocated by Brown-Sequard 

 (1853), which states that the contractions of the uterus are incited by a 

 sudden increase in the carbon dioxid content of the mother's blood. 

 This explanation, however, does not clearly depict the cause of this 

 accumulation, nor is it quite certain that ordinary amounts of carbon 

 dioxid could actually produce this result. More plausible are those 

 theories which localize the stimulus in the fetus itself. Thus, Spiegel- 

 berg has stated that certain of its excretory substances eventually fail 

 to be eliminated and attack the uterus directly. This view finds 



