FCETAL CIRCULATION. 553 



from before backwards ; the face small. The upper extremities are greatly 

 developed, and the thorax expanded and full. The upper part of the 

 abdomen is large, from the great size of the liver ; the lower part is small 

 and conical. And the lower extremities are very small in proportion to 

 the rest of the body. The external genital organs are very large, and fully 

 developed, and the attachment of the umbilicus is one inch farther from 

 the vertex of the head than from the soles of the feet ; and one inch farther 

 from the ensiform cartilage than from the symphysis pubis. 



OSSEOUS SYSTEM. The development of the osseous system has been 

 treated of in the first chapter. The ligamentous system presents no pecu- 

 liarity deserving of remark. 



MUSCULAR SYSTEM. The muscles of the foetus at birth are large and 

 fully formed. They are of a lighter colour than those of the adult, and of 

 softer texture. The transverse stria; on the fibres of animal life are not 

 distinguishable until the sixth month of foetal life. 



VASCULAR SYSTEM. The circulating system presents several peculiari- 

 ties : Istly, In the heart ; there is a communication between the two auri- 

 cles by means of the foramen ovale. 2dly, In the arterial system ; there 

 is a communication between the pulmonary artery and descending aorta, 

 by means of a large trunk, the ductus arteriosus. 3dly, Also in the arterial 

 system ; the internal iliac arteries, under the name of hypogastric and 

 umbilical, are continued from the foetus to the placenta, to which they re- 

 turn the blood which has circulated in the system of the foetus. 4thly, In 

 the venous system ; there is a communication between the umbilical vein 

 and the inferior vena cava, called the ductus venosus. 



FCETAL CIRCULATION. 



The pure blood is brought from the placenta by the umbilical vein. The 

 umbilical vein passes through the umbilicus, and enters the liver, where il 

 divides into several branches, which may be arranged under three heads : 

 Istly, Two or three, which are distributed to the left lobe. 2dly, A single 

 branch, which communicates with the portal vein in the transverse fissure, 

 and supplies the right lobe. 3dly, A large branch, the ductus venosus, 

 which passes directly backwards, and joins the inferior cava. In the in- 

 ferior cava the pure blood becomes mixed with that which is returning 

 from the lower extremities and abdominal viscera, and is carried through 

 the right auricle, (guided by the Eustachian valve) and through the fora- 

 men ovale, into the left auricle. From the left auricle it passes into the 

 left ventricle, and from the left ventricle into the aorta, whence it is dis- 

 tributed, by means of the carotid and subclavian arteries, principally to 

 the head and upper extremities. From the head and upper extremities, 

 the impure blood is returned by the superior vena cava to the right auricle ; 

 from the right auricle, it is propelled into the right ventricle ; and from the 

 right ventricle into the pulmonary artery. In the adult, the blood would 

 now be circulated through the lungs, and oxygenated ; but in the foetus 

 the lungs are 'solid, and almost impervious. Only a small quantity of the 

 blood passes therefore into the lungs ; the greater part rushes through the 

 ductus arteriosus, into the commencement of the descending aorta, where 

 it becomes mingled with that portion of the pure blood which is not sent 

 through the carotid and subclavian arteries. 



Passing along the aorta, a small quantity of this mixed blood is distri 

 47 



