CIRCULATION. 



641 



Fig. 313. 





Foetal circulation seen from behind. 



continuity of tube exists between the maternal 

 and fetal vessels, the blood of the child seems 

 to undergo a respiratory alteration, or a certain 

 degree of arterialization, in being brought into 

 near proximity with the maternal blood. 



The blood of the fetus, after passing through 

 the minute ramifications of the umbilical arte- 

 ries ( U', U) in the placenta, returns by the 

 umbilical vein (11) into its body. 



The umbilical vein carries part of its blood 

 directly by the ductus venosus (d) to the vena 

 cava inferior, and part is distributed by the 

 branches of the vena portae ( L), with which 

 the umbilical vein unites, through the sub- 

 stance of the liver, and is then conveyed by 

 means of the hepatic veins (/) into the general 

 current of the returning blood. 



The right auricle of the heart (/<'), therefore, 

 receives not only the blood which has circu- 

 lated through the body of the foetus, but also 

 that which has passed through the placenta, 

 consequently a mixture of venous and arterial 

 blood ; the blood in the superior vena cava 

 (*) being entirely venous, that in the inferior 

 vena cava ( V) being mixed. The blood which 

 is brought to the right auricle is in much 

 greater quantity in the fetus before birth than 

 in the child which has breathed air ; a part of 

 this blood passes from the right into the left 

 auricle (h) by the foramen ovale (f) in the sep- 

 tum auricularum, and it would ap'pear that it is 

 chiefly the blood from the inferior vena cava 

 which takes that course. 



The rest of the blood entering the right 

 auricle takes the same route as in the adult, 

 viz. into the right ventricle (H'), and thence 

 into the pulmonary artery, but, as very little 

 blood is sent to the collapsed lungs, a passage 



of communication is established in the fetus 

 from the pulmonary artery into the descending 

 aorta through the ductus arteriosus ( D), and 

 thus the greater mass of the blood, which in 

 the adult would have proceeded to the lungs, is 

 in the fetus immediately transmitted to the 

 aorta (A). 



From the disposition of the Eustachian valve, 

 it is believed that nearly the whole of the blood 

 of the inferior vena cava passes from the right 

 to the left auricle through the foramen ovale, 

 while the blood brought from the head and 

 superior extremities (parts which are compara- 

 tively large in the fetal condition) passes 

 through the right side of the heart. The as- 

 cending aorta, rising from the left ventricle, 

 delivers almost all the blood expelled by the 

 contraction of that cavity into the carotid and 

 subclavian arteries, while the ductus arteriosus 

 passing between the trunk of the pulmonary 

 artery and the descending aorta directs the 

 blood which passes through the right ventricle 

 to the lower regions of the body. In this 

 manner the upper regions of the body are sup- 

 plied with the most arterialized part of the 

 blood from the left side of the heart and aorta, 

 while the purely venous blood is propelled 

 from the right ventricle through the pulmonary 

 artery and ductus arteriosus into the descend- 

 ing aorta, and consequently into the lower part 

 of the body, and by the umbilical vessels to the 

 placenta. 



The foramen ovale in the septum of the au- 

 ricles, the ductus arteriosus passing from the 

 pulmonary artery to the aorta, the ductus ve- 

 nosus leading from the umbilical vein to the 

 vena cava inferior, and the umbilical vein and 

 arteries are the structural peculiarities of the 

 fetal circulating organs. These passages are all 

 closed up, and the umbilical vessels obliterated 

 at the navel after aerial or pulmonic respiration 

 is established at birth.* 



II. COURSE OF THE BLOOD IN VARIOUS 



ANIMALS. 



We now leave for the present the history of 

 the circulation in man, in order to give a brief 

 sketch of the varieties of this function in other 

 animals, the study of which is calculated to 

 throw considerable light upon some of the pro- 

 cesses of the human economy, and to illustrate 

 the anatomical and physiological relations of 

 the circulatory and respiratory organs. -f- 



It has been shewn that a regular and pro- 

 gressive circulation of the nutritive fluids occurs 

 in those animals only in which the aeration of 

 the blood is performed by a separate and dis- 



* Sabatier, Mem. de PA cad. An 8. Kilian, 

 Kreislauf im Kinde, &c. Karlshruhe, 1826. Bur- 

 dach's Physiologic, &c. vol. ii. Jeffray, Pecu- 

 liarities of the Postal Circulation. Glasgow, 1834. 



t In the following view of the comparative phy- 

 siology of the circulation, besides the different 

 works referred to under the separate heads, we have 

 been guided chiefly by the following, viz. the works 

 of Cuvier, Home, Meckel, Blumenbach, Trevira- 

 nus, Carus, and R. Wagner ; Roget's Bridgewater 

 Treatise, and the excellent chapter upon this sub- 

 ject by J. Miiller in Burdach's Physiologie, vol. iv. 

 and in his Handbuch der Physiologie, vol. i. 



