448 MANUAL OF ELEMENTARY ZOOLOGY 



and is known as the Fallopian tube. It runs backwards 

 and enlarges into the uterus, a vascular-walled structure 

 which joins its fellow in the middle line in front of the 

 bladder to form the vagina. This passes backwards within 

 the pelvic girdle above the neck of the bladder, with which 

 it presently unites to form the urinogenital canal or vestibule, 

 which opens at the vulva. On its ventral wall lies the small, 

 rod-like clitoris and on the dorsal wall two small Cowper's 

 glands. 



During the spring and summer a periodical ripening 

 of ova with their discharge, or ovulation, occurs at intervals 

 of about a month. Coition takes place during one of these 

 periods, shortly before ovulation. The spermatozoa travel 

 up the oviducts and fertilisation takes place at the upper 

 ends of the latter. The ova pass down the oviducts, in 

 which they segment. At the end of the third day they 

 reach the uterus. Here at first they lie free. On the 

 eighth day, however, they begin to become attached to the 

 uterine wall, and in the course of the next few days there 

 is formed in connection with each of them a special organ, 

 known as the placenta, in which blood vessels derived from 

 the mother and the developing young lie side by side in 

 very close and extensive contact. Through the thin walls 

 of the two sets of blood vessels interchange of fluid and 

 gaseous contents takes place, and in this way the nutrition 

 and respiration of the young is provided for until birth, 

 which takes place at the end of a month from fertilisa- 

 tion. Animals in which, as in the rabbit, a great part 

 of development takes place within the body of the 

 mother, so that the young when they are born are beyond 

 the need of a shell or similar covering, are said to be 

 viviparous. 



The heart of the rabbit lies in the front part of the chest, 

 enclosed in the thin pericardium, immediately 

 Hoart Vo88el8: behind the soft, pink thymus. It has no 

 sinus venosus or conus arteriosus, but there 

 are two ventricles as well as two auricles (atria), so that four 

 chambers are present. Three venae cavse corresponding 

 to those of the frog open directly into the right auricle 

 (Fig. 419), and two pulmonary veins lead by a common 

 opening into the left auricle. The opening from the right 



