SOG HANDBOOK OF PHYSIOLOGY. 



the blood that has circulated in the head and neck and arms, and has 

 been brought to the auricle by the superior vena cava. It might be 

 naturally expected that the two streams of blood would be mingled in 

 the right auricle, but such is not the case, or only to a slight extent. 

 The blood from the superior vena cava the less pure fluid of the two 

 passes almost exclusively into the right ventricle, through the auriculo- 

 ventricular opening, just as it does in the adult; while the blood of the 

 inferior vena cava is directed by a fold of the lining membrane of the 

 heart, called the EustacMan valve, through the foramen ovale into the 

 left auricle, whence it passes into the left ventricle, and out of this into 

 the aorta, and thence to all the body, but chiefly to the head and neck. 

 The blood of the superior vena cava, which, as before said, passes into 

 the right ventricle, is sent out thence in small amount though the pul- 

 monary artery to the lungs, and thence to the left auricle, as in the 

 adult. The greater part, however, by far, does not go to the lungs, but 

 instead, passes through a canal, the ductus arteriosus, leading from the 

 pulmonary artery into the aorta just below 7 the origin of the three great 

 vessels which supply the upper parts of the body; and there meeting 

 that part of the blood of the inferior vena cava which has not gone into 

 these large vessels, it is distributed with it to the trunk and lower parts, 

 a portion passing out by way of the two umbilical arteries to the 

 placenta. From the placenta it is returned by the umbilical vein to the 

 under surface of the liver, from which the description started. 



Changes after Birth. After birth the foramen ovale closes, and so 

 do the ductus arteriosus and ductus venosus, as well as the umbilical 

 vessels; so that the two streams of blood which arrive at the right auri- 

 cle by the superior and inferior vena cava respectively, thenceforth 

 mingle in this cavity of the heart, and passing into the right ventricle, 

 go by way of the pulmonary artery to the lungs, and through these after 

 purification, to the left auricle and ventricle, to be distributed over the 

 body. 



The Nervous System. 



The Cranial and Spinal Nerves. The cranial nerves are derived from 

 a continuous band, called the neural band. They are formed before the 

 neural canal is complete. The neural band is made up of two lamina? 

 going from the dorsal edges of the neural groove to the external epiblast. 

 It becomes separated from the epiblast, and then forms a crest attached 

 to the upper surface of the brain. The posterior roots of the spinal 

 nerves arise as outgrowths of median processes of cells from the dorsal 

 >rtfe of the spinal cord, which become attached laterally to the spinal 

 cord as their original point of attachment disappears. The anterior 

 roots probably arise from the ventral part of the cord as a number of 



