REPRODUCTION. 691 



The Vagina. The vagina is a musculo-membranous canal, from 12 to 

 1 8 cm. in length, situated between the rectum and bladder. It extends 

 from the surface of the body to the brim of the pelvis, and embraces at its 

 upper extremity the neck of the uterus. 



Ovulation. After the establishment of puberty a Graafian follicle 

 develops and ripens or matures periodically, usually every twenty-eight days. 

 During the time of maturation the follicle increases in size, from an augmen- 

 tation of its fluid contents, and approaches the surface of the ovary, where it 

 forms a projection varying from 6 to 12 mm. in size. When maturation is 

 complete the vesicle ruptures, and the ovum and liquid contents are discharged. 

 The ovum, by a mechanism not fully understood, is received by the fimbriated 

 extremity of the Fallopian tube and enters its cavity. The ovum is then 

 transferred through the tube by the peristaltic contraction of its muscle- 

 fibers and by the action of the cilia of its lining epithelium. The time 

 occupied in the transference of the ovum from the ovary to the interior of the 

 uterus has been estimated to be from four to ten days. 



Either at the time of, or very shortly after, its discharge from the follicle, 

 the ovum, and more especially the nucleus, undergoes a series of histologic 

 changes which eventuates in an extrusion of a portion of the chromatin 

 material. The extruded portions are known as the polar bodies. The non- 

 extruded portion of the chromatin material is known as the female pronu- 

 cleus or germ nucleus. The chromosomes are reduced to one-half the somatic 

 number. The succession of changes which the nucleus undergoes is termed 

 maturation. As the nucleus is regarded as the part of the ovum which 

 transmits parental characteristics it is assumed that the extrusion of a por- 

 tion of the nuclear material is a means by which an excess of inherited 

 substance is prevented. 



Menstruation. Menstruation is a periodic discharge of blood and 

 mucus from the surface of the mucous membrane of the uterus, and occurs 

 about every twenty-eight days. The duration of the menstrual period ex- 

 tends over four or five days and the amount of blood discharged varies 

 from 180 c.c. to 200 c.c. Menstruation is usually an accompaniment of 

 ovulation, though the latter process may take place independently of the 

 former. It is characterized by both local and systemic changes. The 

 local changes are most marked in the uterus, the mucous membrane of which 

 increases in thickness from a proliferation of the connective tissue and a 

 hyperemic condition of the blood-vessels. Subsequently to these changes 

 the epithelial surface, as well as the more superficial portions of the connec- 

 tive tissue, undergo degeneration and exfoliation, after which the finer 

 blood-vessels rupture and permit of an escape of blood into the uterine 

 cavity. At the end of the menstrual period regenerative changes set in 

 which continue until the normal condition of the mucous membrane is 

 reestablished. 



The Corpus Luteum. With the rupture of the Graafian follicle there is 

 an effusion of blood into the follicular cavity which soon coagulates, loses its 

 color and assumes the characteristics of fibrin. The walls of the follicle, 

 which have become thickened from the deposition of a reddish-yellow glutin- 

 ous substance, now become convoluted and undergo a still further hypertrophy, 

 until they encroach upon and almost obliterate the follicular cavity. In a 



