THE MAMMALIAN SEXUAL CYCLE 59 



with this the secondary sexual characters become accentuated ; 

 there is an acceleration of growth and an increase of strength ; 

 in the boy there is a growth of hair on the face and other parts of 

 the body, and enlargement of the larynx and consequent deepen- 

 ing of the voice, processes which are not completed until about 

 the twenty-fifth year. In the girl the pelvis widens at puberty, 

 and the subcutaneous layer of fat w^hich assists so largely in giving 

 the body its graceful contour is deposited. In both sexes there 

 are correlated psychical changes. 



The menstrual cycle comes to a final end at the menopause 

 or climacteric at an average age of forty-five, or rather earlier 

 among the inhabitants of hot climates. Atrophic changes then 

 take place in the ovaries, uterus, and mammary glands, and 

 ovulation no longer occurs. Sexual desire also abates, though 

 it may be very marked during the change, and it may persist for 

 some time afterwards. The change lasts from three to five years, 

 menstruation being at first irregular and then finally ceasing. 

 The organic functions are especially irregular during the post- 

 cessation stage. Palpitation, dyspepsia, sweating, and vaso- 

 motor changes are common, and these are sometimes associated 

 with hysteria, neurasthenic symptoms and mental instability. 

 After the change is completed the metabolism settles down on 

 a new level, and the various organs become once more adjusted 

 so as to permit of a normal existence. 



In the male there is no climacteric, sexual capacity declining 

 gradually. It is know^n, however, that spermatozoa may continue 

 to be produced in small numbers even in extreme old age, and that 

 insemination may still be successfully performed, though the 

 chances of a union being sterile are undoubtedly increased. 



Among animals there is a period of puberty or sexual maturity 

 just as in man, the spermatozoa and ova being ripened then for 

 the first time. Among the domestic animals fillies will generally 

 come " in use " when eighteen months old, cows at ten months, 

 sheep and pigs at six, and dogs and cats at ten months, or some- 

 what earlier. Mice will breed at six weeks, rats at two months, 

 and rabbits at five months. Female animals in a state of nature 

 generally die before reaching the climacteric, but if kept in 

 captivity or under domestication for a sufficiently long time 

 reach a condition of permanent sterility, after which they may 

 still retain health and a measure of vigour. 



