r 



CHAP. XLIII.] UMBILICAL CORD. 623 



embryo, as it does in many animals ; but this statement has been 

 completely refuted by the researches of Muller, Bischoff, Langen- 

 beck, and others. 



Umbilical Cord. The umbilical cord is the long, narrow pedicle, 

 contained in a tube of the amnion, which connects the foetus with 

 the placenta. In the advanced embryo, it consists principally of 

 the large vessels, through the intervention of which all the nutrient 

 material absorbed from the blood of the mother is conducted to 

 the system of the foetus. 



At an earlier period of development, the cord is really composed 

 of 



1. The remains of the omphalo-enteric duct, or pedicle of the 

 umbilical vesicle. 



2. The vasa omphalo-meseraica, or branches of the mesenteric 

 vessels of the foetus. 



3. The urachus, and all that remains of the allantois. 



4. The umbilical vessels ; consisting of one umbilical vein, which 

 brings the blood back from the placenta, and two umbilical arte- 

 ries, by which the blood is carried to the placenta. 



In animals generally, however, there are two veins, as well as 

 two arteries, which are the chief branches of the hypogastric arte- 

 ries of the foetus. The circulation of the foetus has been fully 

 described in page 348. 



Birth. In the human subject, the period of pregnancy lasts 

 about nine solar, or ten lunar, months, or 280 days. It varies, 

 however, within certain limits, 



The phenomena of parturition are specially treated of in works 

 on Midwifery ; so that a very brief reference to this part of the 

 subject will only be required. 



Of the immediate cause of the contraction of the uterus, little is 

 known. Valentin attributes it to the excitement of the organs 

 which always exists at the menstrual periods ; and he considers 

 that parturition takes place at the tenth menstrual period. Dr. 

 Tyler Smith has advocated a similar view, and believes that the 

 contractions of the uterus are due to the increased action of the 

 ovaries operating upon the cord through the ovarian nerves, which 

 act as excitors ; while the uterus is thrown into contraction 

 through the medium of the uterine nerves, which are therefore to 

 be regarded as the motor nerves concerned in this reflex action. 

 The action of the uterus is, no doubt, in part due to the stimulus 

 produced by the increasing bulk of its contents. 



The foetus lies in utero with its head downwards during the later 



