148 OBSTETRICAL PHYSIOLOGY. 



that an animal is pregnant, but no physical indication reveals this state, 

 the examinations must be repeated before a final decision is arrived at. 

 The value of experience and observation, when added to theoretical 

 knowledge, is well displayed in this branch of veterinary science. Not 

 unfrequently the diagnosis of pregnancy is surrounded by great diffi- 

 culties, and a guarded opinion must therefore always be given. 



SECTION rV'.— DURATION OF PREGNANX'Y. 



The duration of pregtiancy varies considerably in different species 

 of domesticated animals ; and even in the same species there are 

 individual variations which, though not very great, are yet important ; 

 so that the exact term cannot be rigorously fixed. From the doe 

 Eabbit, which carries its young only twenty-eight to thirty days, to the 

 female Elephant, whose period of pregnancy is, according to report, 

 two years, there are a number of intermediate terms ; and it is scarcely 

 possible to establish a satisfactory relationship between the duration of 

 pregnancy and the other conditions of organisation — such as size or 

 longevity. In closely allied species, and which may couple and pro- 

 duce hybrids — as the Horse and Ass, Sheep and Goat — the period is 

 pretty nearly the same. 



With regard to breeds, Wilhelms has remarked that the Hungarian 

 Cow averages ten days more than the Dutch Cow. With the male 

 foetus, the duration of gestation is greater than in the case of a female. 

 It has even been remarked that the male parent may have an influence 

 in this direction. For instance, a Mare which has been put to a 

 thoroughbred Horse will be longer pregnant than when impregnated by 

 a common-bred stallion ; and the Mare which has been fecundated by 

 a stallion Ass goes longer than when impregnated by a Horse. The 

 duration of pregnancy also depends upon the age of the female, and 

 her strength and condition ; a weakly or much-worn animal does not 

 go so long as one which is strong and weU fed. 



The differences in indi\'iduals of the same breed or species may be 

 partly accounted for by the fact, that impregnation is possible at any 

 time during oestrum — a variable period ; and if coitus has taken place 

 several times during this condition, it is impossible to predict when 

 conception took place. And even when contact has only occurred once 

 between the male and female, fecundation does not necessarily coincide 

 with this intercourse ; as the ovum may meet the spermatozoa in 

 different regions of the uterine system, and may therefore only be ferti- 

 lised some days after copulation. The time required for the o\'um to 

 pass through the Fallopian tube also varies in different animals. In 

 the Eabbit and Guinea-pig, for instance, it takes three days ; in Eumi- 

 nants from four to five days, and in the Bitch from eight to ten days. 



And, as has been remarked, various circumstances may retard or 

 accelerate the development of the foetus ; not only this, but with some 

 animals it may apparently remain for a number of days in the uterus 

 after it is ready for birth, without inconvenience to the mother or itself, 

 just as it may be born several days before the ordinary period without 

 compromising its safety. 



For these reasons, the period of pregnancy can only be approximately 

 fixed, though there are of course limits beyond which Nature, ever 

 provident and watchful for the preservation of species, cannot go without 

 ceasing to be natural. 



