PHYSIOLOGY. 29, 
The temper becomes altered. It occasionally happens 
that Mares which are vicious at any other period, are 
perfectly quiet at this; while others again are very 
troublesome when in this condition. 
Certain changes also take place in the ovaries. 
These organs become congested and considerably 
enlarged. One or more Graafian vesicles become fully 
developed, find their way to the surface of the ovary, 
and when there, rupture, their contents escaping into 
the Fallopian tubes. In animals which give birth to 
only one at a time, only one Graafian vesicle is 
ruptured, and that generally in the right ovary; but in 
those that have several young at a birth, two or more 
vesicles are ruptured, the rupture taking place from 
right and left alternately. When these changes in the 
female are going on and an ovum has been liberated, 
if no contact with the male is allowed, impregnation 
can not take place (except as before stated), and the 
various organs resume their normal condition, and 
remain so for a time, which varies in the different 
animals. 
In the Cow, the period of cestrum is present from 
one to four days, and then disappears, to reappear in 
from twenty-one to twenty-two days. In the Mare, the 
period lasts from three to six days, and disappears, to 
return again in from twenty-one to twenty-two days. 
The Bitch is in heat generally twice a year, in 
early spring and autumn, and the period lasts from 
eight to fifteen days, 
In Great Britain, the Sheep appears in heat about 
the middle of September, the condition lasting for 
