478 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



increased labor upon both the liver and the kidneys, and these organs are prone 

 to functional disorders. Gastric disturbances are marked by frequent vomit- 

 ing. A tendency to increased pigmentation in the skin is present. The ner- 

 vous system is affected, manifesting its alterations both by nutritional disturb- 

 ances and by mental irritability, depression of spirits, disordered senses, easily 

 passing into temporary pathological states, and occasionally by feelings of 

 heightened well-being. The body-weight usually increases independently of 

 the added weight of the embryo. 



Duration of Gestation. — For centuries the duration of gestation in 

 woman has been commonly regarded as 280 days. The beginning of preg- 

 nancy, the union of the ovum and the spermatozoon, however, presents no 

 obvious signs by which it may be recognized, and hence the actual length of 

 pregnancy in the human female is no more known than in other mammals. 

 The obstetrician is obliged, therefore, to use artificial schemes in computing its 

 probable length. Several tables have been published of the time elapsing 

 between a single coition resulting in pregnancy and the terminal parturition. 

 Veit, 1 in collecting 503 such cases reported by several obstetricians, finds the 

 duration to be from 265 to 280 days in 396 cases, and longer in the remaining 

 107 cases, the variation thus being marked. It is obvious that the date of the 

 effective coition can rarely be known. One of the first and most evident signs 

 of pregnancy is the non-appearance of the menses, and, probably largely from 

 the long-prevailing idea of the close relation existing between ovulation and 

 menstruation, it has been customary to regard gestation as dating from the last 

 menstruation. Following Naegele, obstetricians estimate the date of parturi- 

 tion as 280 davs from the first day of the last menstruation; and this simple 

 but artificial rule is doubtless approximately correct. 



In accordance with modern biological theories, it must be supposed that for 

 each species there has been developed a gestative period of a length most 

 favorable to the continuance of the species; this has been a matter of natural 

 selection. But this principle does not account for the termination of the period 

 in any individual case. The proximate cause of the oncoming of birth must 

 be sought in more specific anatomical or physiological phenomena. This cause 

 has been sought long, and not wholly successfully. Among the agents sug- 

 gested may be mentioned the pressure which the uterine tissues, the ganglia 

 of the cervix, and the adjacent nerves, receive between the fetal head and the 

 pelvic wall, the stretching of the uterine wall, the fatty degeneration of the 

 decidual, the thrombosis of the placental vessels, the venosity of the fetal 

 blood due to the growing functional importance of the fetal right ventricle 

 acting asa stimulus to the placental area, and a gradual increase in irritability 

 of the uterus as the nerve-supply of the organ increases. Some of these, such 

 as the fatty degeneration of the deciduse and the placental thrombosis, are no! 

 constant phenomena, and the others are not definitely proved to be efficient 

 causes. It i> probable that, with the uterus undoubtedly irritable, in different 



1 J. Veit : Midler's Handbuch der QeburtohiUfe, 1888, 1. 



