482 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



nearly all of them go to the inferior mesenteric ganglia and thence by the 

 hypogastric nerves to the uterus. Stimulation of the uterus itself, the vagina, 



the vulva, the sciatic ami the crural nerves, and various sensory regions, 

 notably the nipples, causes reflex contractions of the uterus. The same result 

 occurs upon stimulation of various portions of the brain, such as the medulla 

 oblongata, the cerebellum, the pons, the corpora quadrigemina, the optic thala- 

 mus, the corpus striatum, and even the corpus callosum. In woman psychic in- 

 fluences may call forth or inhibit uterine contractions. How largely the well- 

 known stimulating effects of the blood in asphyxia and of drugs, like ergot, 

 are due to central, and how largely to direct uterine influence is undecided. 

 The regular co-ordinated course of labor and many experimental facts make 

 it probable that, normally, reflex influences constitute a large part of the proc- 

 ess, the centripetal impulses arising within the uterus itself, probably largely 

 from the pressure upon the walls of the lower segment and the cervix. 

 In fact, it is customary to speak of labor as a complex reflex action. The 

 undoubted automatism of the uterine muscle-fibres must, however, be taken 

 into account, and the act should be regarded as composed of both automatic 

 and reflex elements. We have here to deal with that variety of contractility 

 peculiar to smooth muscle, in which central and peripheral influences work 

 together to bring about the result. It is perhaps not going too far to regard all 

 such actions, like that of the heart, as primarily automatic and called out In- 

 direct stimulation, but as modified and controlled by reflex influences. The 

 parturitive contractions of the striated muscles of the abdominal walls are 

 probably more generally reflex in nature, modified, however, by voluntary 

 effort s. 



Multiple Conceptions. — According to the records given by different stat- 

 isticians, the frequency of twin births varies considerably in different coun- 

 tries. In 13,000,000 births in Prussia, G. Veit ' found the number of twins 

 to be 1.12 per cent., or 1 in 89 births. In the cities of New York and 

 Philadelphia recent reports give the ratio of twins to single births as 1 : 120, 

 or 0.83 per cent. 



Observations of discharged Graafian follicles in cases of multiple concep- 

 tions show that twins may arise either from separate eggs or from a single egg. 

 The presence at birth of a double chorion is commonly regarded as diagnostic 

 of the former origin, that of a single chorion of the latter. In the former 

 case the two ova may come from a -ingle Graafian follicle, or from two folli- 

 cles situated within one ovary, or from both ovaries, direct observation of the 

 ovaries themselves being required to determine the origin in any particular 

 case. The two ova are discharged and fertilized probably at approximately 

 the same time. There are two distinct amnion.-. The two placentas may be 

 either fused into one or wholly separated from each other, and accordingly the 

 decidua reflexa may be single or double. The two offspring may be of sep- 

 arate sexes, and do not necessarily closely resemble each other. In cases 

 where the two embryos come from a single ovum their origin is little under- 

 1 G. Veit: Monatsschrift fur Geburtekunde und FrauenkrankheUen, L855, vi. 



