THE THORAX. 221 



on leaving the same it is laden with O. The difference, then, between the ante 

 partem and post partem purification of blood is not a physiological one ; but a 

 difference is to be found in certain anatomical routes taken by this blood, in the 

 one case to and from the placenta, in the other case to and from the lungs. A 

 study of the metamorphosis of parts after functional activity ceases is replete 

 with interest, since it shows the converse of the law of physiology that growth 

 is the correlation of function. Study the following steps in the foetal circulation, 

 in order that in your dissection you may be able to appreciate the several fcetal 

 vestiges you shall find there. 



1. In the foetus the blood is purified in the placenta of the mother. (Fig. 

 155.) It is taken to that organ by the hypogastric arteries, from the internal 

 iliac arteries. 



2. It is returned from the placenta by the umbilical vein, as follows : (i) To 

 the ascending vena cava by the ductus venosus ; (2) to the portal vein to traverse 

 the liver, and finally reach the ascending vena cava by the hepatic veins. 



3. The blood from above the diaphragm reaches the heart by the descending 

 vena cava, as in the adult. 



4. The blood from the descending vena cava passes through the auricle to 

 the ventricle, and thence out through the pulmonary artery ; the lungs not being 

 yet fitted for purifying, the blood is taken by the ductus arteriosus to the aorta. 



5. The blood from the ascending vena cava passes through the foramen ovale 

 (Fig. 154) to the left auricle, thence to the left ventricle, and out by the aorta. 



The circulatory apparatus of the fcetus possesses : 



1. The umbilical vein brings blood from the placenta. 



2. The hypogastric arteries conveys blood to the placenta. 



3. The ductus venosus takes blood to the vena cava. 



4. The ductus arteriosus connects the pulmonary artery and aorta. 



5. The foramen ovale is an opening between the auricles. All of these 

 structures forming the circulatory apparatus of the fcetus are set aside at birth, 

 when the lungs become the aerating organ for the blood. By non-use these 

 structures lose their specific characters they become obsolete. They obtain in 

 the adult as : 



1. Remains of the umbilical vein, the round ligament of the liver. 



2. Remans of the hypogastric arteries near the urachus. 



3. Remains of the ductus venosus on under surface of liver. 



4. Remains of the ductus arteriosus between aorta and pulmonary artery. 



5. Remains of the foramen ovale in posterior wall of auricle. 



What is a fcetal vestige? The fibrous remains in the adult of an organ that 

 under the conditions of the fcetus in utero was a necessary organ or part. The 

 umbilicus and urachus are foetal vestiges. 



What are the grand divisions of the thorax / 



The thorax may be subdivided into a pulmonary part, occupied by the lungs, 

 and a mediastinal part, not occupied by the lungs. 



Is the word thorax synonymous with trunk / 



No ; thorax is the cavity above the diaphragm ; trunk means the whole body 

 except the head, neck, and extremities. 



Explain the superior mediastinal space. 



This is a space bounded above by the superior aperture of the thorax ; below 

 by a plane limited in front by the manubrio-gladiolar articulation and behind by 

 the body of the fourth thoracic vertebra ; in front by the manubrium ; behind by 

 the four upper thoracic vertebme ; laterally by the pleural sacs. This space 



