350 MECHANISMS OF THE GENERATIVE APPARATUS. 



6-5 cms. long, at the end of pregnancy it is 35 cms. in length by 24 

 cms. in width, and is twenty-four times as heavy as the uniinpregnated 

 uterus. This growth is mainly due to the hypertrophy of the muscular 

 tissues, the larger muscular cells being increased seven- to eleven-fold in 

 length, and two- to five-fold in breadth. Moreover, a number of fresh 

 muscle cells are developed, apparently from cells which have the 

 character of embryonic muscle cells. At the same time there is a 

 marked growth in the mucous membrane which forms the decidua, and 

 in all the connective tissue structures of the uterus, including blood 

 vessels and lymphatics. 



Along with the growth of the ovum, the uterus increases in size 

 and becomes more distended, its irritability becoming greater, so that 

 it is easily excited to contract. The stimulus may be supplied from 

 adjacent abdominal organs, from the brain as by emotions, or by direct 

 excitation of the internal surface of the uterus, in consequence of move- 

 ments of the foetus. In many cases no antecedent stimulus can be 

 discovered, and the automatic contraction of the uterus seems to be 

 analogous to that which occurs in the distended bladder. These con- 

 tractions ordinarily give rise to no sensations, and are only felt when 

 they are augmented in consequence of reflex stimulation. During the 

 greater part of pregnancy they have little or no effect on the contents 

 of the uterus. During the last weeks or days of pregnancy, however, 

 these contractions, which have now become more marked, have a dis- 

 tinct physiological effect. Not only do they, by pressing on the foetus, 

 cause it, in most instances, to assume a suitable position for its sub- 

 sequent expulsion, but, affecting the whole body of the uterus, including 

 the longitudinal muscular fibres surrounding its neck, they assist the 

 general enlargement of the organ in dilating the internal os, so that the 

 upper part of the cervix is obliterated and drawn up into the body of 

 the uterus some little time before labour has commenced. 



With these changes in the uterus are associated changes in the 

 round ligaments and in the vagina and vulva. The muscular fibres of 

 the round ligaments become much hypertrophied and lengthened, and 

 these structures can therefore aid appreciably the uterine contractions 

 in the subsequent expulsion of the foetus. The vaginal walls become 

 thickened and of looser texture, so as to afford less resistance to 

 distension during the passage of the foetal head. 



Considerable discussion has taken place as to the cause for the onset 

 of the processes comprised under the heading of labour or parturition, 

 at a nearly constant period of two hundred and seventy-two days after 

 conception. Most of the explanations which have been suggested, 

 such as the great irritability of the uterus at the termination of 

 pregnancy, the loosening of the foetal membranes, the return of the 

 menstrual congestion after ten months, thrombosis of the placental 

 sinuses, simply replace one question by another. According to Spiegel- 

 berg, the phenomena accompanying the birth of twins, which are often 

 born at a considerable interval from each other, the onset of con- 

 tractions of the uterus at the right time in normal as well as in extra- 

 uterine foetation, the fact that the extra-uterine foetus dies when it 

 has become mature, all go to show that the reason why labour occurs 

 at a definite time must be sought for in foetal rather than in uterine 

 changes. This author suggests that some substances which had pre- 

 viously been used up by the foetus gradually accumulate in the maternal 



