128 COMPARATIVE PHYSIOLOGY. 



Mare, ass, zebra 11 months. 



Camel 13 



Giraffe 14 



Elephant 23 to 25 " 



The period of gestation in the human subject is nine months ; 

 in the monkeys and apes somewhat less. The incubation pe- 

 riod of certain of our domestic birds and related species is about 

 as follows : 



Canary 14 days. 



Pigeon T- 18 



Hen 21 " 



Duck, goose, pea-hon 38 " 



Guinearhen 25 



Turkey 38 '• 



It is interesting to note that the smaller varieties of fowls 

 hatch out sooner than the larger ; and that the period of incu- 

 bation of all of the above varies with the weather, the steadi- 

 ness of the incubating bird, the date on which the eggs selected 

 were laid, etc. With very recent eggs, an attentive sitter, and 

 warm weather, the incubation period is shortened. 



PARTUBITION. 



AU 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 a solution. The foetus lies surrounded 

 (protected) by fluid contained in the amniotic sac. For its ex- 

 pulsion there is required, on the one hand, a dilatation of the 

 uterine opening {os uteri), and, on the other, an expulsive force. 

 The latter is furnished by the contractions of the uterus itself, 

 aided by the simultaneous action of the abdominal muscles. 

 Throughout the greater part of gestation the uterus experiences 

 somewhat rhythmical contractions, feeble as compared with the 

 final ones which lead to expulsion of the foetus, but to be re- 

 garded as of the same character. With the growth and func- 

 tional development of other organs, the placenta becomes of 

 less consequence, 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 ex- 



