XIIl] FEMALE GENERATIVE ORGANS 191 



not acting and there being no 3'oung ones in process of passing 

 out from tlie uterus to propel the last pigling through the vaginal 

 opening. 



Abortion or premature parturition may occur at any time 

 during gestation. There are two kmds, sporadic and contagious 

 abortion. The former may occur as a result of undue muscular 

 exertion, fatigue, injury, excitement or fright, or it may be 

 caused by illness (e.g. digestive trouble or the taking of frozen 

 food) or by eating ergot or other substances which act upon the 

 uterine muscles. Contagious abortion is due to a bacillus and is 

 esiDecially common in cattle; it may be transmitted by the bull 

 or the contagion may be sj)read through contaminated litter, the 

 baciUi probably entering through the vagina and j^assing into the 

 uterus. Immunity, at any rate for a certain period, is acquired 

 through an attack. Contagious abortion also occurs in mares, and 

 occasionally in sheep and pigs but is due usually to different 

 organisms which are in some degree specific for the animals 

 attacked. 



Lactation. The mammar}^ glands which have been already 

 described, commence to secrete at or shortly before iDartvu-ition, 

 the thin colostrum which at first appears rapidly giving place to 

 true milk. The capacity for milk production varies widely in the 

 different breeds, some cows going on yielding until the approach 

 of a new parturition before going dr3\ The drawing off of milk 

 is itself a factor in the peld, since as is weU known, a 'quarter' 

 A\ill go dry if the milk secreted is not removed by milking or 

 sucking; in such cases the milk constituents are reabsorbed and 

 lactose ma}^ often be found in the urine. The physiological factors 

 in the growth of the glands and the secretion of the fluid are 

 referred to below in dealing with the ovarian secretions. 



Fecundity. The number of 3'oung produced at a litter is 

 dependent primarily upon the number of ova discharged at 

 ovulation which occurs during oestrus. With the mare and cow 

 this number is generalh' one, but double and even triple births 

 are not unknown. With the ewe one or two young are born at a 

 time, while triplets are not uncommon, and four or five lambs have 

 occasionall}^ been produced at one time. The sow may give birth 

 to anj' nmnber up to 16, and occasionalK' more, a litter of 24 

 having been recorded. The bitch has been known to produce as 

 many as 23 pups, but an}' number higher than 12 is very rare; an 



