42 THE PHYSIOLOGY OF REPRODUCTION 



characteristics, as is shown, for example, by the Dorset Horn sheep 

 in the South of England, and still more evidently by the Limestone 

 sheep of Westmorland and Derbyshire. But that an increase in the 

 duration (or more frequent recurrence) of the sexual season is not 

 necessarily a highly artificial condition or the result of special 

 attention in regard to food supply, etc., on the part of the flock- 

 master, is shown by such a condition occurring among the indigenous 

 sheep of India and the half-wild sheep of Patani. 



The duration of the dioestrous cycle in Blackfaced sheep, as 

 already mentioned, is from about thirteen to twenty-one days, the 

 variation appearing to depend partly upon the nature of the country 

 in which they live. In other breeds the cycle may be said to vary 

 within approximately the same limits. Ellenberger, 1 however, gives 

 from twenty to thirty days as the length of this interval. The 

 procestrum and oestrus together do not as a rule occupy more than 

 two or three days, and oestrus alone may last for only a few hours. 

 The external signs of the prorestrum are comparatively slight in 

 sheep. 2 The vulva is usually somewhat congested, and there is often 

 a flow of mucus from the external generative aperture, but blood is 

 seldom seen. Owing to the extreme shortness of the "heat" period 

 the mucous flow may continue during the oestrous and metoestrous 

 periods. The internal changes are briefly described in the succeeding 

 chapter. The only external indication of oestrus is that afforded by 

 the behaviour of the ewes. At this time they tend to follow the ram, 

 and display a general restlessness of demeanour. The period of 

 gestation is twenty-one or twenty-two weeks. Nathusius' observa- 

 tions show that it is fairly constant within the limits of particular 

 breeds. 3 



The oestrous cycle in the sheep, and its great variability, have 

 been discussed at some length, since this animal is probably typical 

 of most Ungulata in the way in which its generative system is 

 affected by different conditions of life, while the facts about other 

 Ungulates are not so perfectly known. The effect of changed 

 conditions upon the sheep's fertility, i.e. upon its capacity to bear 



1 Ellenberger, Verqleichende Physiologic der Hamsaiiqethiere, vol. ii., Berlin, 

 1892. 



2 It is interesting to note that Aristotle clearly distinguished between 

 the prooestrum and oestrus in the sheep and goat. This is what he says : 

 " With ewes and she-goats there are signs of menstruation in breeding time, 

 just before the time for submitting to the male ; after copulation also the signs 

 are manifest, and then cease for an interval until the period of parturition 

 arrives" (Thompson's Translation, loc. cit.). 'The "signs after copulation" doubt- 

 less refers to vaginal bleeding, such as has been observed by Mr. Hammond 

 in cows at such a time. 



3 Nathusius, " Ueber einen auffallenden Racemmterschied in der Trachtig- 

 keitsdauer der Schafe," Zool. Garten, Jahrg. 3, 1862. (Cf. p. 68.) 



