182 PHYSIOLOGY OF FARM ANIMALS [CH. 



occtirrence of pregnancy) may contiiuie for half the yesir or even 

 longer but there is much individual variation. The period of gesta- 

 tion is nine months. Usually only one follicle is discharged at 

 oestrus. Heat may supervene six weeks after parturition. Pro- 

 oestrum and oestrus together only occupy two or three da3^s. 



In the soAv the dioestrous cycle is also three weeks and preg- 

 nancy lasts four months. By weaning the young pigs early and 

 giving the sow a plentiful supply of food it is possible to obtain 

 five litters in two jea,TS, but the more usual practice is to have 

 two litters annually. Heat maj' be experienced four Aveeks after 

 farrowing and in the absence of the boar continue to recur at 

 intervals of three weeks, each oestrous jDcriod lasting about a day. 

 The indications of prooestrum are slight, there being generally no 

 external bleeding, but the vulva is swollen. 



In all the above-mentioned animals oestrus is a period of 

 restlessness and excitement. The ewe on heat tends to follow the 

 ram. The cow or heifer bellows and mounts other cows, and some- 

 times there is a slight rise of body temperature. The sow emits a 

 peculiar grunting sound which is characteristic of oestrus and 

 is not made at other times. But as compared with the bitch 

 the symptoms of T)estrus are slight and of brief duration. 



The mare is pol yoestro us, the normal dioestrous cycle being 

 about three weeks and the oestrous period four or five days, though 

 its actual length may vary by three or four days. The sexual 

 season in the absence of the stallion extends throughout the spring 

 and early summer months, and is generally longest in the more 

 domesticated breeds. Ewart has recorded a case of a pony im- 

 ported from Timor (which is in the Southern Hemisphere), to 

 Scotland, in which oestrus was experienced in the autumn, or at 

 the same time as the spring in Timor. The period of gestation in 

 the mare is eleven months, and 'heat' recurs from nine to eleven 

 days after parturition. This is called the 'foal heat.' Certain 

 mares are irregular in the recurrence of the 'heat' periods, and, 

 in some, 'foal heat' does not occur until seventeen days after 

 parturition instead of the usvial time. In exceptional cases a mare, 

 like a cow, may conceive at the ' foal heat ' and yet take the horse 

 three weeks later, just as though she had failed to become preg- 

 nant. Heape states that, very excei^tionally, mares are monoe- 

 strous. Blood has been observed in the mare's prooestrous discharge, 

 but it is not generally present. The external generative organs, 



