Not only the cells of the cervix but also the cells of the vagina produce a mucus which is made up of membranous glycoproteins. In the vagina there are intermediate cells which are able to contribute to the presence of mucus on infertile days. A reason for the very frequent appearance of a flow outside the vagina during those days is a reduction of the reabsorption by the pockets of Shaw (Figures 1 and 14). The pockets are situated in the inferior part of the vagina and the contribution of vaginal mucus emanates from the superior part. In this case a gynaecological examination is quite normal and cytological examination shows more intermediate cells. Examination of a slide by microscope also shows quantities of F mucus in the cervical "spread out" specimens.

Figure 14. Cervical anatomy and mucus and fluid flow during fertile and infertile phases of the cycle. The locations of crypts P, S, L, and G are indicated as also are F cells (F); I, isthmus (secretion Z); Can cerv, cervical canal; End, endometrium; L. maj., Labium major; L. min., Labium minor; Ov, ovary; Per, perinium; Poc. Shaw, pocket of Shaw; Rec, rectum; Sym, symphesis; Tro, tube; Ure, urethra; Vag, vagina; Ves, bladder.

Normally the reabsorption by the pockets of Shaw is reduced in the fertile phase because of the thickening of the preovulatory vaginal epithelium (Odeblad 1989). The capacity for reabsorption during adolescence may be retarded and dry days may be absent. If a young woman commences to take the Pill, maturation of the pockets is even more retarded. Forthcoming papers will give more detailed presentations of the role of the vagina for the Ovulation Method.