REPRODUCTION. 479 



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

 several of 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 pf 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 oh 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, arc 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 

 and introduced into the uterus. During the earlier part of parturition the 

 contraction- gradually increase in intensity up to a maximum which they 

 then maintain. Their rhythm is somewhat irregular \ the duration of each 

 contraction averages about one minute, and a pause, which ensues between suc- 

 cessive contractions, extends from one and one-half to several minutes. The 

 relaxation of the muscle-fibres during the period of rest is incomplete, the 

 result being that the fibres enter gradually into a tonieally contracted state. 

 Each contraction is accompanied by a pain, localized in the early pari of labor 

 in the uterus alone, but later extending outward, upward into the abdomen, 

 and downward into the thighs. The pains of labor vary greatly in intensity 

 in individuals, but are usually more intense during the firsl gestation than 

 during later ones. They are due chiefly to direct mechanical stimulation of the 

 sensory uterine and other nerves by compression, tension, and even laceration. 



1 V. Schatz : Archiv fur Uyuil koloyir, INN") S(i, xwii. < ompare O. Schaeffer : Experimentelle 

 Untersuchungen iiier die Wehentkatigkeii den menschlichen Uterus, auagefiihrt miitelst einer neuen 

 Pelolte iokI eines neuen Kymoyraphion f Berlin, 1896; abstract in Centralblatt fur Gynalcologie, 

 1896, xx. 8. 85. 



