518 SANGUIFEROUS SYSTEM. 



as we ascend from fishes to man, has not yet reached the 

 middle of the trunk of the body. 



The various grades of development in the sanguiferous 

 system, thus traced throughout the animal kingdom, are 

 successively represented in the transient forms which this 

 system assumes during its development in all the higher 

 animals and man. The colourless blood of the embryo at 

 first moves through the germinal membrane in two conti- 

 guous circles, like that of the lowest annelides ; and when, at 

 length, vessels are distinctly formed, the pale red blood con- 

 tinues to circulate in closed arteries and veins, without the 

 aid of a heart, as in all the radiated and many higher tribes 

 of animals* The pulsating, heart-forming centre of this 

 system, becomes, as in worms and insects, a dilated dorsal 

 vessel; and the punctum saliens forms a muscular ventri- 

 cle, which developes an auricle behind it, and a bulbus ar- 

 teriosus on its anterior part. Beyond this bulb, the 

 aorta early divides into five successive pairs of deciduous 

 branchial arteries, and the embryonic branchial open- 

 ings on the sides of the neck have been retained to ma- 

 turity, as an abnormal condition, in the human body. 

 The cavities of the heart lose their primitive rectilineal posi- 

 tion, which they retain in the gasteropods ; the auricle gra- 

 dually doubles up behind the base of the ventricle, and 

 arrives at that dorsal position, with regard to it, which it 

 retains in fishes; the two ends of the primitive, conti- 

 nuous, heart-forming, dorsal vessel, are now become posterior 

 vena cava and aorta, where the blood still moves through 

 the cavities of the heart, as in insects, from behind forwards. 



The three anterior branchial arteries, forming simple 

 aortic arches in man, as in mammalia, birds, and rep- 

 tiles, are successively converted into the ascending trunks 

 from the arch of the aorta. The fourth arch forms the 

 descending aorta, and the posterior arch the pulmonary 

 arteries, as during the metamorphosis of a frog. So 

 that man's vascular system, arrived at the possession 

 of a single muscular ventricle, represents that of the 

 highest Crustacea ; with an auricle and ventricle placed 

 in a line, it becomes that of a gasteropod ; and with a 

 developed bulbus arteriosus, an auricle advanced upon 

 the ventricle, and the aorta divided into branchial arches, 

 it is raised to that of fishes the embryos of all 



