'=998 OF GENERATION. 



!^een found, by the experiments of Dr. J. Reid, 1 to serve the purpose of direct- 

 jng the arterial blood, which flows upwards from the ascending Cava, through 

 the foramen ovale, into the left auricle, whence it passes into the left ventricle ; 

 whilst it also directs the venous blood, that has been returned by the descending 

 Cava, into the right ventricle. When the ventricles contract, the arterial blood 

 which the left contains is propelled into the ascending Aorta, and supplies the 

 branches that proceed to the head and upper extremities, before it undergoes 

 any admixture ; whilst the venous blood, contained in the right ventricle, is 

 forced through the Pulmonary artery and Ductus Arteriosus into the descend- 

 ing Aorta, mingling with the arterial current which that vessel previously con- 

 veyed, and passing thus to the trunk and lower extremities. Hence the head 

 and superior extremities, whose development is required to be in advance of 

 that of the lower, are supplied with blood nearly as pure as that which returns 

 from the placenta ; whilst the rest of the body receives a mixture of this with 

 what has previously circulated through the system ; and of this mixture a por- 

 tion is transmitted to the placenta, to be renovated by coming into relation with 

 the maternal fluid. At birth, the course of the current is entirely changed by 

 its diversion into the Lungs, which takes place immediately on the first inspi- 

 ration. The Ductus Venosus and Ductus Arteriosus soon shrivel into liga- 

 ments; the Foramen Ovale becomes closed by its valve; and the circulation, 



Fig. 269. 



An embryo Dog, representing the junction of the umbilical vesicle with the intestinal canal : a, rudiment- 

 ary nostrils; b, rudimentary eyes; c, the first visceral arch; d, the second visceral arch ; e, the right,/, the 

 left auricular appendage ; g, the right, h, the left ventricle of the heart ; i, the aorta ; k, the liver, between 

 the two lobes of which is perceived the divided orifice of the omphalo-mesenteric vein ; I, the stomach ; m, the 

 intestine, communicating with the umbilical vesicle ; n, o, the Wolfiian bodies ; p, the allantois ; q, the upper 

 extremities ; r, the lower extremities. After Bischoff. 



"Edinb. Med. and Surg. Journal," vol. xliii. ; and " Anat., Physiol., and Pathol. Re- 

 searches," Chap. ix. 



