54 THE PHYSIOLOGY OF REPRODUCTION 



become irregular with advancing age, and this irrespectively of 

 whether or not the animal is permitted to become pregnant. The 

 periodicity depends also to some extent upon climate, for in 

 Danish Greenland the dogs usually breed only once a year.^ 



The procBstrum in the bitch is characterised externally by the 

 vulva being swoUen and moistened with mucus, and by the 

 existence, usually, but not absolutely invariably, of a flow of 

 blood from the aperture of the vagina. The length of the 

 procestrura is about ten days. The sanguineous discharge 

 generally ceases at the commencement of oestrus, which may 

 last for another week or ten days. 



Heape states that the winter oestrus in some breeds does not 

 last so long as the summer oestrus. In certain individuals a 

 relatively shght mucous or sanguineo-mucous flow takes place 

 during the period of oestrus, and may even be continued beyond 

 it, but this is exceptional. Stonehenge states that a bitch will 

 not, as a rule, receive the dog until external bleeding has sub- 

 sided, and that the most favourable time for successful coition 

 is about the eleventh day of " heat " (in' other words, at the 

 beginning of the period of oestrus). This statement is fully' 

 borne out by dog-breeders. 



The external changes which occur during " heat " are 

 accompanied by changes in the metabolism, for Potthast,^ 

 working on the nitrogen metabohsm of the bitch, records a 

 slight retention of nitrogen during the " heat " period. A 

 similar result was obtained by Hagemann,^ who states that the 

 retention is followed by a loss of nitrogen after copulation. 

 These results should be compared with those recorded for 

 menstruating women (see p. 68). 



The histological changes which take place in the uterus 

 during the oestrous cycle are described in the next chapter. 



The period of gestation in the dog varies from fifty-nine to 

 sixty-three days. With dogs belonging to the smaller breeds 

 the period is often somewhat less than with large dogs. The 



' Rink, Danish Greenland, London, 1877. 



' Potthast, " Kenntniss des Eiweissumsatzes," Dissertaiion, Leipzig, 

 1887. 



' Hagemann, " Eiweissumsatz im tierlsoh Organismus," Dissertation, 

 Erlangen, 1891. Cf. also Schbrndorff, " Einfluss der Schilddruse auf den 

 StofEwechsel," Pfiiiger's Arch., vol. Ixvii., 1897. 



