122 COMPARATIVE PHYSIOLOGY. 



eration, they are thrown ofE and renewed at these periods {cata- 

 menia, menses, etc.), provided pregnancy does not take place. 

 In mammals helow man, in their natural state, pregnancy does 

 almost invariably take place at such times, hence this exalted 

 activity of the mucous coat of the uterus, in preparation for the 

 reception and nutrition of the ovum, is not often in vain. In 

 the human subject the menses appear monthly ; pregnancy may 

 or may not occur, and consequently there may be waste of na- 

 ture's forces ; though there is a certain amount of evidence that 

 menstruation does not wholly represent a loss; but that it is 

 largely of that character among a certain class of women is 

 only too evident. As can he readily understood, the catamenial 

 flow may take place prior to, during, or after the rupture of the 

 egg-capsule. 



As the uterus is well supplied with glands, during this 

 period of increased functional activity of its lining membrane, 

 mucus in considerable excess over the usual quantity is dis- 

 charged ; and this phase of activity is continued for a time should 

 pregnancy occur. 



All the parts of the generative organs are supplied with 

 muscular tissue, and with nerves as well as blood-vessels, so 

 that it is possible to understand how, by the influence of nerve- 

 centers, the various events of ovulation, menstruation, and 

 those that follow when pregnancy takes place, form a related 

 series, very regular in their succession, though little prominent 

 in the consciousness of the individual animal when normal. 



In aU animals, without exception, the disturbance of the 

 generative organs during the rutting season (oestrum) is accom- 

 panied hy unusual excitement and special alterations in the 

 temper and disposition, while it may perhaps be said that the 

 whole organism is correspondingly affected. 



The frequency of the season of heat or rutting is variable, as 

 also its duration. In most of the domestic animals it lasts but a 

 few days; though in the bitch it may be prolonged for a 

 month. 



It is not common for conception to occur in the human sub- 

 ject whUe the young one is being suckled, and the same remark 

 applies to the domestic animals, though less so, and with con- 

 siderable variation for different species. 



Naturally, the periods of oestrum will depend considerably 

 on the occurrence of impregnation and the duration of gesta- 

 tion. It is usual for the mare to be in season in spring and fall. 



