464 OF GENERATION AND DEVELOPMENT. 



and partially elaborated by the two sets of intervening cells ; and in 

 this character, the foetal tufts resemble the villi of the intestinal surface, 

 which dip down into the fluids of the alimentary canal, and absorb the 

 nutritive material which they furnish. But the Placenta also serves 

 as a respiratory organ ; aerating the blood of the foetus, by exposing it 

 to the influence of the oxygenated blood of the Mother ; and in this 

 respect the foetal tufts bear a close correspondence with the gills of 

 aquatic animals, bringing the blood into relation with a surrounding 

 fluid medium containing oxygen, which is imbibed by the blood in 

 exchange for the carbonic acid given off. And it is probable, too, 

 that the Placenta is to be regarded as an excreting organ ; serving for 

 the removal, through the maternal blood, of excrementitious matter, 

 whose continued circulation in the blood of the foetus would be prejudi- 

 cial to it. 



821. The formation of the Human Placenta commences in the latter 

 part of the second month of utero-gestation ; during the third, it 

 acquires its proper character ; and it subsequently goes on increasing, 

 in accordance with the growth of the ovum. The vessels of the Uterus 

 undergo great enlargement throughout ; but especially at the part to 

 which the Placenta is attached; and the blood, in moving through 

 them, produces a peculiar murmur, which is usually audible with dis- 

 tinctness at an early period of pregnancy, and which may be regarded 

 (when due care is taken to avoid sources of fallacy) as one of its most 

 unequivocal physical signs. The sound is most commonly heard near 

 the situation of the Fallopian tube of the right side ; and it corresponds 

 with the pulse of the mother. 



822. It would be inconsistent with the character and objects of this 

 Treatise, to follow, in any detail, the history of the development of the 

 Foetus, during its intra-uterine life ; and a general account of the evo- 

 lution of most of the chief organs, has been given in connexion with 

 that of their structure. The condition of the Circulating apparatus, 

 however, at the period of birth, deserves especial notice. A general 

 account of the development of the simple pulsating trunk, which con- 

 stitutes its first form, into the four-cavitied heart of the higher Verte- 

 brata, and of the conversion of the single trunk proceeding from it, 

 with its four pairs of branchial arches, into the aorta and pulmonary 

 arteries, with their chief subdivisions, has been already given ( 566). 

 Up to the time of birth, however, the partition between the Auricles is 

 incomplete ; a large aperture, the foramen ovale, existing in it. There 

 is also a direct communication between the pulmonary artery and the 

 aorta, by the ductus arteriosus ; and another direct channel between 

 the umbilical vein and the vena cava, by the ductus venosus. 



823. The following is the course of the Circulation of the Blood in 

 the Foetus. The fluid brought from the Placenta by the umbilical vein 

 (Fig. 150, 3), is partly conveyed at once to the vena cava ascendens, by 

 means of the ductus venosus (5), and partly flows through two trunks 

 (4, 4), that unite with the portal vein (7) returning the blood from the 

 intestines, into the substance of the liver, thence to be returned to the 

 vena cava by the hepatic vein. Having thus been transmitted through the 

 two great depurating organs, the placenta and the liver, the blood that 



