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A MANUAL OF VETERINARY PHYSIOLOGY 



out foetal life, pouring their secretion into special depressions 

 in the placenta (Assheton). 



The Duration of Pregnancy appears to be based on no fixed 

 law. Judging from the length of time the elephant is in gesta- 

 tion, it might appear that body size had an influence, but against 

 this is the fact that the ass carries her young longer than the 

 horse, while, whether it be a toy terrier or a Newfoundland, a 

 dog goes from fifty-nine to sixty-three days. It certainly does 

 appear that among animals of the same species breed has an 

 influence in the matter ; different herds of cows vary from 277 

 to 288 days, Merino sheep average 150 days, Southdowns 144 

 days. It is not clear why, of two rodents, the guinea-pig should 

 require a period of gestation twice as long as the rabbit. 



The following are average periods of gestation : 



Parturition. — The foetus having reached its full stage of develop- 

 ment, changes of an obscure nature take place which lead to its 

 expulsion. During uterine life the equine foetus is lying on its back 

 on the floor of the mother's abdomen, with its chin on its chest, 

 the fore-legs bent at the knee, and the hind-legs in the right horn 

 (Fig. 254). Preparatory to birth the foetus changes position and 

 turns on its side, so as to assume, first a lateral position (Fig. 255), 

 and lastly an upright one (Fig. 256), by which the foetal and maternal 

 spines are brought nearer together. To assume this position the 

 foetus has had to make a complete revolution ; it is now brought 

 with the muzzle and fore-legs in the direction of the pelvis (Fig. 256), 

 and dilatation of the passage follows. In the cow the foetus lies 

 on its back on the floor of the abdomen as in the mare, but somewhat 



* Tessier, in a memoir on the period of gestation presented in 1825 to 

 the Royal Academy of Science in Paris, stated that in 582 mares, after 

 one sexual congress, the shortest period of gestation was 287 days, the 

 longest 419 days, a difference of 132 days. In 312 mares which visited 

 the horse several times, the shortest period of gestation, calculated from 

 the last cover, was 290 days, the longest 377 days, a difference of 87 days. 



f Tessier gives the period of two asses, one being 12 months 20 days, 

 the other 13 months 1 day ; and these agree very closely with Sutherland's 

 observations (Veterinary Record, October 13, 1894), quoted in the text 

 above. 



I Of 1,131 cows, the average period observed by Tessier was 240 days ; 

 the longest was 321 days (Veterinarian, vol. i., 1828). 



