298 ANIMAL PHYSIOLOGY. 



certain amount of variation. Toward the end of that time, a 

 great relaxation of parts takes place, even the ligaments 

 which bind the bones of the pelvis being considerably slack- 

 ened; the cervix uteri begins to dilate, and the muscular 

 walls of the uterus are seized with recurrent spasmodic 

 contractions. Usually the uterine contractions burst the 

 membranes which retain the liquor amnii, and afterwards 

 the child is expelled, its head first, and is shortly followed 

 by the placenta and ruptured membranes. In healthy par- 

 turition, the contraction of the uterus is sufficient to pre- 

 vent much bleeding ensuing from the tearing across of the 

 blood-vessels which united the placenta with its substance. 

 Forthwith, the uterus begins to undergo a rapid process of 

 involution. Its muscular walls, which had undergone great 

 enlargement, both by increased size of the individual fibres, 

 and continual growth of others from connective-tissue cor- 

 puscles, become daily diminished by degeneration and dis- 

 appearance of the exaggerated fibres; and the organ returns 

 to its original proportions. 



218. Within a day or two after the birth of the child, the 

 breasts of the mother, which have previously been enlarging, 

 come into functional activity, and there is a copious secretion 

 of milk. 



The breasts, or mammce, have their proper secreting struc- 

 ture, at other times, so closely connected with the areolar 

 tissue in which it is imbedded that it is difficult to trace; 

 but during lactation, it is much more distinct. From about 

 fifteen to twenty separate ducts open at the extremity ol 

 the nipple; each of these is dilated into a small reservoir 

 between one and two inches from its extremity, and, traced 

 further back, is found to branch repeatedly, till the radicles 

 are reached, connected with the ultimate lobules, which con* 

 sist of aggregations of rounded secreting saccule's. ' 



Milk, examined microscopically, is a clear fluid, with oil 

 globules of many different sizes floating in it. In the process 

 of churning, the oil globules of milk are thrown together in 

 a solid mass, and constitute butter; while in milk which is 

 allowed to remain at rest, so that the larger globules rise to 

 the top without running together, the stratum in which these 

 accumulate is the cream. The solid constituents of milk are 



