6go A MANUAL OF VETERINARY PHYSIOLOGY 



days, and is followed by oestrus, the returning heat usually lasting 

 longer by two or three days than the original ' heat.' 



The cow under domestication will breed at any time of the 

 year (Goodall). She ordinarily takes the bull six weeks or two 

 months after calving, but it is unusual for her to accept the bull 

 while suckling her calf. If the latter be removed or weaned, 

 she shows signs of oestrus six or seven days later, the duration of 

 which may be twelve hours. The period of dicestrum is twenty- 

 one days, at the end of which time both cows and heifers exhibit 

 oestrus. This cycle continues until they are settled in calf. 



With sheep* oestrus may only last one or two days, or it may 

 pass away very quickly, the dicestrum which follows lasting 

 from thirteen to eighteen days. The number of recurrent periods 

 in any one cycle in the sheep have been observed to depend upon 

 breed ; two, three, or four recurrent periods have been noted. 

 There are some breeds of sheep which may produce two sets of 

 lambs in one year. The period of oestrus may be induced almost 

 at any time in the late summer and autumn by the introduction 

 of the ram to the ewes (Goodall). 



The sow takes the boar about a week after she has weaned 

 her litter, or about eight weeks after farrowing. The period of 

 oestrus lasts about two days, the dicestrous period twenty-one 

 days. 



The only known animal which in a wild state exhibits a con- 

 tinuous series of dicestrous cycles is the monkey, but even in 

 this case the season is limited when conception is possible (Heape) . 



Cause of CEstrus. — The oestrus period may appear in the dog 

 after a portion of the spinal cord has been excised, proving that 

 it is a process quite independent of any reflex act, that it may 

 exist in the absence of any knowledge on the part of the animal, 

 and that its production is under no central control. Further- 

 more, such an animal may become pregnant and be delivered 

 in the ordinary way, though quite unconscious of the process. 

 (Estrus and menstruation are produced as the result of an 

 internal secretion derived from the stroma of the ovary (see 

 Corpus Luteum, p. 698) . 



The External Signs of Procestrum in all animals are a swelling 

 of the vulva, more or less pronounced, with a slight flow of 

 mucus, which may be blood-stained. There is excitement, The 

 mare may refuse to work, squeals and kicks when approached, 

 elevates and protrudes the clitoris, and micturates frequently, the 

 material being very mucoid. The cow bellows, is excited, and 

 mounts its fellows. Sheep become restless and follow the ram. 

 The dog is playful, excited, and desires the attention and com- 



* ' The CEstrous Cycle and Function of the Corpus Luteum in the 

 Sheep,' by F. H. A. Marshall, B.A., Phil. Trans., B., vol. cxcvi., 1903. 



