THE FEMALE ORGANS 351 



semen into the vagina is due to the ischio-cavernosus 

 and bulbo-cavernosus muscles, which undergo rhythmic 

 contraction. 



The centre controlhng ejaculation hes in the lumbo- 

 sacral region of the cord. The efferent fibres for the con- 

 traction of the vasa deferentia leave the cord by the second, 

 third and fourth lumbar roots. Passing through the in- 

 ferior mesenteric gangha, they form the hypogastric nerves. 

 The motor fibres to the ischio-cavernosus and bulbo- 

 cavernosus muscles lie in the nervi erigentes. 



THE FEMALE ORGANS OF REPRODUCTION 



Between puberty, when the sexual organs first be- 

 come active, and the menopause or climacteric (at about 

 the forty-sixth year), when they cease to function, the 

 ovary and uterus undergo a parallel series of cyclical 

 changes, which are only interrupted by the more profound 

 modifications which occur during pregnancy. 



The cycle of changes, which is fundamentally the same 

 in all mammals, is known as the OGstrous cycle. lif consists 

 of the following phases : — - 



Pro-oestrum. — This is the period of uterine congestion, 

 culminating in a discharge of blood and mucus. 



CEstrus or Period of Desire.— This follows immediately 

 upon pro-cestrum. In many mammals it is the only 

 period during which the female evinces sexual desire and 

 during which copulation leads to fertilisation. O^^strus 

 corresponds in point of time to ovulation, to be described 

 later. 



Metoestrum. — During this period the activity of the 

 sexual organs diminishes. 



Ancestrum or Period of Rest.— The sexual organs are now 

 relatively quiescent. 



Some animals— e. 5^. rabbit, bitch — may, after coitus, 

 undergo a condition known as pseudo-pregnancy — the 

 uterus, mammary gland and corpora lutea (see below) 



