REPROD UCTION. 917 



each species there has been developed a gestative period of a length most 

 favorable to the continuance of the species ; this has been a matter of natural 

 selection. But this principle does not account for the termination of the period 

 in any individual case. The proximate cause of the oncoming of birth must 

 be sought in more specific anatomical or physiological phenomena. This cause 

 has been sought long, and not wholly successfully. Among the agents sug- 

 gested may be mentioned the pressure which the uterine tissues, the cervical 

 ganglion, and the adjacent nerves, receive between the fetal head and the pelvic 

 wall, the stretching of the uterine wall, the fatty degeneration of the deciduae, 

 the thrombosis of the placental vessels, the venosity of the fetal blood due to 

 the growing functional importance of the fetal right ventricle acting as a 

 stimulus to the placental area, and a gradual increase in irritability of the 

 uterus as the nerve-supply of the organ increases. Some of these, such as the 

 fatty degeneration of the deciduse and the placental thrombosis, are not con- 

 stant phenomena, and the others are not definitely proved to be efficient causes. 

 It is probable that, with the uterus undoubtedly irritable, in different cases 

 different stimuli act to inaugurate the process of birth, and a priori the above 

 causes seem not improbable ones. 



Parturition in General. Parturition, birth, or labor, is the process of 

 expulsion of the developed embryo, the membranes, and the placenta from the 

 body of the mother. It is executed by contraction of the muscles of the so- 

 called upper segment of the uterus and those of the abdominal walls. The 

 lower segment of the uterus, comprising approximately that portion of the 

 body lying below the attachment of the peritoneum, the cervix, the vagina, 

 and the vulva, are largely, if not wholly, passive in parturition. The obstet- 

 ricians have found it convenient to divide labor into three stages, although 

 physiologically these are not sharply differentiated from each other. The first 

 stage is characterized by the dilatation of the os uteri, the second by the expul- 

 sion of the fetus, the third by the expulsion of the after-birth. The customary 

 position of the fetus within the uterus at the end of pregnancy is that in which 

 the head is downward or nearest the os, the back toward the ventral and left 

 side of the mother, and the arms and legs folded upon the trunk. 



First Stage of Labor. For several weeks toward the close of pregnancy 

 there are occasional periods when rhythmic muscular contractions pass over the 

 uterine walls. These are mostly painless, and apparently are not in themselves 

 of special functional importance. The first stage of labor is ushered in by 

 various phenomena, prominent among which are an increase in the intensity 

 of the contractions, their painfulness, and their frequency and continuance. 

 In women they are confined practically to the upper segment of the uterus and 

 its attached ligaments, ceasing at a circular ridge that projects inward and is 

 called the "contraction ring." For some reason, at present disputed, the 

 lower segment of the uterus, and the cervix, are passive. The contractions 

 are probably peristaltic in character, as in lower animals. Schatz 1 has graphi- 

 cally recorded the uterine movements by means of a bladder filled with water 

 1 F. Schatz : Archiv fur Gyniikologie, xxvii., 1885-86. 



