720 OF REPRODUCTION. 



943. A change in the type of the Circulating system of the foetus, from 

 that at first presented by it ( 940), takes place at a very early period. At 

 about the 4th week, in the Human Embryo, a septum begins to be formed in 

 the Ventricle; and by the end of the 8th week it is complete. The Septum 

 Auriculorum is formed at a somewhat later period, and it remains incomplete 

 during the whole of foetal life ; it is partly closed by the valvular fold cover- 

 ing the Foramen Ovale, which fold is developed during the third month. 

 During the same period, a transformation takes place in the arrangement of 

 the large vessels proceeding from the Heart; which ends in their assumption 

 of the form they present until the end of Foetal life; and this undergoes but 

 a slight alteration, when the plan of the circulation is changed at the moment 

 of the first inspiration. The number of Aortic arches on each side, which 

 was five at first, soon becomes reduced in the Mammalia to three, by the 

 obliteration of the two highest pairs. The Bulbus Arteriosus is subdivided, 

 by the adhesion of its walls at opposite points, into two tubes, of which one 

 becomes the Aorta and the other the Pulmonary Artery ; and of the three 

 pairs of (branchial) arches, the highest, being connected with the Aortic trunk, 

 contributes to the formation of the Subclavian and Carotid arteries ; whilst of 

 the middle pair, the arch on the right side is obliterated, the other becoming 

 the Arch of the Aorta. The lowest pair arises from the Pulmonary trunk, 

 and forms the Pulmonary artery on each side ; that on the left side, however, 

 goes on to join the descending Aorta as before, and thus constitutes the Ductus 

 Arteriosus. 



944. The following is the course of the circulation of the blood in the 

 Foetus. The fluid brought from the Placenta by the Umbilical Vein is partly 

 conveyed at once to the Vena Cava ascendens, by means of the Ductus 

 Venosus, and partly flows through the Vena Portae into the Liver, whence it 

 reaches the ascending Cava by the Hepatic Vein. Having thus been trans- 

 mitted through the two great depurating organs, the Placenta and the foetal 

 Liver, it is in the condition of arterial blood ; but, being mixed in the vessels 

 with that which has been returned from the trunk and lower extremities, it 

 loses this character in some degree by the time that it arrives in the Heart. 

 In the right Auricle, which it then enters, it would be also mixed with the 

 venous blood conveyed by the descending Cava; were it not that a very curi- 

 ous provision exists, to prevent (in great degree, if not entirely) any such fur- 

 ther dilution. The Eustachian valve has been found, by the experiments of 

 Dr. J. Reid,* to serve the purpose of directing the arterial blood, which flows 

 upwards from the ascending Cava, through the Foramen Ovale, into the left 

 Auricle, whence it passes into the Ventricle ; whilst it also directs the Venous 

 blood, that has been returned by the descending Cava into the right Ven- 

 tricle. When the Ventricles contract, the Arterial blood which the left con- 

 tains is propelled into the ascending Aorta, and supplies the branches that 

 proceed to the head and upper extremities, before it undergoes any admix- 

 ture ; whilst the Venous blood, contained in the right Ventricle, is forced 

 through the Pulmonary artery and Ductus Arteriosus into the descending 

 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 portion is transmitted to the Placenta, to be renovated by coming into rela- 

 tion with the maternal fluid. At birth, the course of the current is entirely 



* Eclinb. Med. and Surg. Journal, vol. xliii. 



