INNERVATION OF THE GENERATIVE ORGANS 577 



of pseudo-pregnancy, which, as we have seen, is dependent on the 

 persistence of the corpus luteum, it is reasonable to suppose that the 

 processes associated with actual parturition after true pregnancy are 

 correlated similarly with changes in the ovarian internal secretions. 

 Ancel and Bourn l have supposed that in the first part of pregnancy 

 the tolerance of the uterus for the foetus is due to the corpus luteum, 

 and in the second half that it is caused by the myometrial gland 

 (see p. 618), but Hammond 2 has shown that it is much more likely 

 that the corpus luteum is the responsible organ throughout the whole 

 of gestation. Further, Sharpey Schafer 3 has shown that the ovaries 

 produce two hormones (see p. 621), one inhibiting the contractility 

 of plain muscle and the other increasing it. Similarly, Guggisberg, 4 

 by injecting extract of corpus luteum into the extirpated uterus, in 

 some cases obtained increased contraction, in others relaxation. It 

 seems probable that whereas during pregnancy the inhibitory 

 secretion is produced in greatest abundance, at the end of pregnancy 

 the secretion which has an excitatory effect on uterine or other 

 muscle may be dominant in its influence, and so promote the act 

 of giving birth and the processes which are associated with it. 



The causes of abortion or premature parturition are disciissed 

 below in the chapter on fertility. 



PROLONGED GESTATION 



The duration of gestation in any one species usually varies within 

 quite narrow limits, but under exceptional circumstances it may 



1 Ancel and Bouin, " Sur le Determinisme de PAccouckement," (J. R. de 

 I'Acad. des Sciences, vol. cliv., 1912. 



- Hammond, " On the Causes Responsible for the Developmental Progress 

 of the Mammary Glands, etc.," Proc. Ray. Soc., B., vol. Ixxxix., 1917. 



3 Sharpey Schafer, The Endocrine Organs, London, 1917. Itagaki states 

 thatf extract of corpus luteum generally produces an increase of tone in the 

 uterus, but sometimes the opposite effect is brought about. This difference is 

 not due to condition of pregnancy or non-pregnancy, nor to the strength of 

 the extract, but to differences in the samples of corpus luteum. Extract may 

 produce relaxation in other muscular tissues. Liquor folliculi caused increase 

 in tone of uterine and other muscles. Extract of hilum ovarii (containing 

 interstitial cells) caused inhibition of uterine muscle, but increase of tone in 

 intestine. ("The Influence of Corpus Luteum Extracts upon Plain Muscle," 

 and "On the Action of Various Extracts, etc.," i^uar. Jour. Exp. Pht/sioL, 

 vol. xi., 1917.) See also Athias, who describes the effects of various extracts 

 of internally secretory organs upon uterine movements, especially after 

 ovariotomy. (" Effets de la Castration sur les Mouvements Automatiques de 

 1'Uterus chez le Cobaye," Jour, de Phys. et de Path. Gen., vol. xviii., 1919 ; 

 Arch. Intemat. de Pharm. et de Ther., vol. xxv., 1920.) The rhythmical 

 contractions after ovariotomy become enfeebled and eventually die away, but 

 the " tonic " oscillations persist. 



4 Guggisberg, "Ueber die Wirkung der inneren Sekrete auf die Tatigkeit 

 des Uterus," Zeitsch. f. Gel. und Gyn., vol. Ixxv., 1913. Extracts of pregnant 

 uterus or of placenta caused contraction. 



19 



