254 THE PHYSIOLOGY OF THE 



is the sole support of life. The urachus, which leads 

 from the fundus of the unborn's bladder to the allantoid 

 cells or sacs, snaps at birth, and that part which is retained 

 by the foal dries up, and becomes the long ligament of the 

 bladder. The chorion, which is the most vascular of the 

 three membranes, as well as the most outward, in the mare 

 is intimately connected with the inner mucous lining of the 

 uterus of the mother. This connexion is formed by nume- 

 rous minute vascular projections upon each, which exactly 

 fit into one another ; those developed upon the chorion of 

 the foetus precisely suiting little spaces left upon the much 

 thickened, and very much more vascular, lining of the gravid 

 uterus of the parent. 



Fcetal circulation. — It has been shown that the placenta 

 owns a maternal and a foetal portion, in which distinct cir- 

 culations are carried on. In both the arteries terminate in 

 capillaries which end in veins. It is evident, therefore, that 

 the blood received, after having circulated through the foetal 

 body, and having given out its nutritive principles, must 

 return venoiis to the placenta, through the umbilical arte- 

 ries ; which, in this case, carry venous blood, as the umbi- 

 lical veins bear blood of a more arterial character. Thus, 

 therefore, the placenta forms the true foetal lungs ; and 

 the likeness is further heightened by the arteries carrying 

 venous blood, and the vein bringing back arterial blood, 

 as in the true pulmonic circulation. The umbilical vein, 

 it w^as just now stated, carries arterial blood, w^hich it de- 

 rives from the placenta: having travelled up the um- 

 bilicus, it leaves the umbilical cord, and proceeds to the 

 vena portse, there to deposit its contents. From the liver 

 the blood, mingled with the venous blood from the in- 

 testines, is passed into the vena cava, and thence to the 

 right auricle ; but it does not wholly enter the right ven- 

 tricle, as in the adult horse ; a part of it escapes through 

 an opening in the septum of the heart, called the foramen 

 ovale, which hes between the right and left auricles ; the 

 blood brought by the posterior cavse is consequently partly 

 poured into the left auricle, whence it is prevented from 

 returning by the Eustachian valve : this opening begins to 

 close up as soon as birth takes place, and respiration is 

 estabUshed. The remaining and smaller portion of the 



