CH. LVIII.] DEVELOPMENT OF THE NERVOUS SYSTEM. 84! 



trunk and other 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. Immediately after birth the foramen 

 ovale begins to close, and so do the ductus arteriosus and ductus 

 venosus, as well as the umbilical vessels ; the closure is com- 

 pleted in a few days, so that the two streams of blood which 

 arrive at the right auricle by the superior and inferior vena cava 

 respectively, thenceforth mingle in this cavity of the heart, and 

 passing into the right ventricle, go by Avay of the pulmonary 

 artery to the lungs, and thence, after purification, to the left 

 auricle and ventricle, to be distributed by the aorta to the body 

 generally. 



Development of the Nervous System. 



The central nervous system originates from the thickened 

 walls of the medullary groove, which by the meeting of the 

 dorsal ridges is converted into the medullary canal. These walls 

 are composed entirely of epiblast. The anterior part of this mass 

 of epiblast becomes the brain, the rest of it the spinal cord ; the 

 canal itself is seen in the adult as the ventricles of the brain and 

 central canal of the spinal cord. The nerves are formed of 

 epiblast too, they are outgrowths from masses of cells called 

 neuroblasts, the primitive nerve-cells. In the case, however, of 

 the olfactory and optic nerves we have not to deal with solid 

 outgrowths, but with hollow protrusions from the brain, which 

 become solid at a later stage. 



The Spinal Cord. The cavity formed by the closure of the 

 neural canal soon becomes a cleft running from before backwards. 

 It is bounded at first by columnar epithelium; these cells afterwards 

 become ciliated ; on their exterior is a homogeneous basement 

 membrane. The wall soon becomes thicker, and the basement 

 membrane is thus separated further and further from the central 

 canal. This increase iu thickness is due in part to the increase 

 in length of the columnar cells : in part to the appearance of new 

 colls. The inner part of the columnar lining retains its palisade- 

 like character, and forms ultimately the lining epithelium of the 

 central canal. The cells are called spon</ioU<uts. The external 

 ends of the cells break up into a reticulum called the myelo- 

 spongium, and this is limited externally by the basement membrane 

 at the circumference. The myelospongium forms the neuroglia. 



