696 HANDBOOK OF PHYSIOLOGY. 



through the pulmonary artery to the lungs, and thence to the left auri- 

 cle, 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 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 umbili- 

 cal arteries to the placenta. From the placenta it is returned by the 

 umbilical vein to the under surface of the liver, from which the descrip- 

 tion 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. (See Chapter oh Circulation.) 



The Nervous System. 



The Cranial and Spinal Nerves. The cranial nerves are derived 

 from a continuous band, called the neural band. They are formed be- 

 fore the neural canal is complete. The neural band is made up of two 

 laminae going from the dorsal edges of the neural groove to the external 

 epi blast. 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 side of the spinal cord, which become attached laterally to the 

 spinal cord as their original point of attachment disappears. The ante- 

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

 strands for each nerve. They appear later than the posterior roots. 

 The rudiment of the posterior root is differentiated into a proximal round 

 nerve connected to the cord, a ganglionic portion and a distal portion. 

 To the last the anterior nerve-root becomes attached. 



The Spinal Cord. The spinal cord consists at first of the undifferen- 

 tiated epiblast of the walls of the neural canal, the cavity of which is 

 large, with almost parallel sides. The walls are at first composed of 

 elongated irregular nucleated columnar cells, arranged in a radiate man- 

 ner. The cavity then becomes narrow in the middle and of an hour-glass 

 shape (Fig. 482). When the spinal nerves make their first appearance, 

 about the fourth day in the chick, the epiblastic walls become differen- 

 tiated into three parts: (a) the epithelium lining the central canal; (b) the 



