152 PRINCIPLES OE VBTEEINAEY SCIENCE 



the blood circulating in the vessels of the dam and that of the 

 fetus. 



The fetal membranes surround the fetus like an envelop and 

 insure protection, nutrition, and union with the mother. They 

 develop early and contain a quantity of fluid to prevent the fetus 

 against sudden changes in temperature and mechanical injuries. 

 During parturition this fluid lubricates the genital canal and 

 expedites the passage of the fetus. There are three fetal mem- 

 branes, the amnion, allantois, and chorion. The amnion is the 

 inner membrane and is in direct contact with the fetus. The 

 chorion is the outermost. When the fetus is expelled from the 

 womb these membranes may be retained; collectively they are 

 then known as the "afterbirth." A complete description of 

 these structures may be found in works on anatomy, embry- 

 ology, or obstetrics. 



Pregnancy and gestation are synonymous terms and refer to 

 the period of time during which the young is developing in the 

 womb. Physiologic pregnancy is the condition that begins 

 with fertilization of the ovum in the fallopian tube and normally 

 ends at parturition when the fetus is born. It is the most re- 

 markable and highly interesting process in nature and exerts 

 important influences upon the mother. Fortunately, nature 

 makes adequate provision for its successful completion. Only 

 when accident or disease interferes does pathologic pregnancy 

 occur. 



An animal that has never given birth to offspring is termed 

 nulliparous; one pregnant for the first time is primiparous; one 

 producing a single offspring at a time is uniparous; one producing 

 two at a birth is biparous; and one producing more than two is 

 multiparous. 



In the case of twins the fetuses may occupy one or both uterine 

 horns and part of the body of the uterus. When confined to a 

 single horn, one twin usually has the anterior and the other the 

 posterior presentation. If both fetuses arise from the same ovum 

 a common chorion is present, but each has its allantois and amnion. 

 When fetuses develop from different eggs, each has its own chor- 

 ion. According to some authorities, while there are temporary 

 deviations in the position of the fetus or fetuses in the uterine 

 cavity, during pregnancy they always return to their original 

 normal positions, which is in the long axis of the horns. In the 



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