120 ANIMAL PHYSIOLOGY. 
riod—the nourishment of structures on the way to a higher 
path of progress. 
As embryonic maturity is being reached, preparation is made 
for a new form of existence; so it is found that the Eustachian 
valve is less prominent and the foramen ovale smaller. 
PARTURITION. 
All the efforts that have hitherto been made to determine 
the exact cause of the result of that series of events which make 
up parturition have failed. This has probably been owing to 
an attempt at too simple asolution. The foetus lies surrounded 
(protected) by fluid contained in the amnioticsac. For its expul- 
sion there is required, on the one hand, a dilatation of the uter- 
ine opening (0s uteri), and, on the other, a visa tergo. The lat- 
ter is furnished by the contractions of the uterus itself, aided by 
the simultaneous action of the abdominal muscles. Through- 
out the greater part of gestation the uterus experiences some- 
what rhythmical contractions, feeble as compared with the 
final ones which lead to expulsion of the foetus, but to be regard- 
ed as of the same character. With the growth and functional 
development of other organs, the placenta becomes of less con- 
sequence, and a fatty degeneration sets in, most marked at the 
periphery, usually where it is thinnest and of least use. It does 
not seem rational to believe that the onset of labor is referable 
to any one cause, as has been so often taught; but rather that it 
is the final issue to a series of processes long existing and grad- 
ually, though at last rapidly, reaching that climax which seems 
like a vital storm. The law of rhythm affects the nervous sys- 
tem as others, and upon this depends the direction and co-ordi- 
nation of those many activities which make up parturition. 
We have seen that throughout the whole of foetal life changes 
in one part are accompanied by corresponding changes in oth- 
ers; and in the final chapter of this history it is not to be ex- 
pected that this connection should be severed, though it is not 
at present possible to give the evolution of this process with 
any more than a general approach to probable correctness, 
CHANGES IN THE CIRCULATION AFTER BIRTH. 
When the new-born mammal takes the first breath, effected 
by the harmonious action of the respiratory muscles, excited 
to action by stimuli reaching them from the nerve-center (or 
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