DEVELOPMENT. 197 



What changes occur in the circulation after the birth of the 



foetus ? 



The respiratory centre in the medulla, which has been quiescent 

 because it has been sufficiently well supplied with oxygen, is 

 awakened as soon as the connection with the uterine sinuses is 

 interrupted. As soon as the supply of oxygen sinks to a certain 

 point, an impulse of inspiration is generated, and the infant breathes 

 and the lungs assume a condition of partial expansion. With the 

 diminished resistance in the expanded lungs the amount of blood 

 in the pulmonary circulation increases, and the amount passing 

 through the ductus arteriosus diminishes, and this is soon obliterated. 

 At the same time, the blood returning to the left auricle increases 

 in quantity, and the intra-auricular pressure is greater ; then, too, 

 the inferior vena cava. sends less blood, for the ductus venosus no 

 longer carries the blood from the placental circulation, and there- 

 fore the foramen ovale is not used, and is soon closed by the 

 adhesion of its valve-like curtain. Thus we have the adult circu- 

 lation established in the place of the foatal in consequence of the 

 respiratory movements. 



How is the spinal cord formed? 



It will be remembered that the medullary canal encloses in its 

 cavity cells from the epiblast which line it. These cells by pro- 

 liferation and differentiation develop nerve-cells and nerve-fibres, 

 the latter at first not medullated. The cells also gradually close in 

 upon the medullary canal, and form a central canal lined with 

 epithelium, a layer of nerve-cells (gray matter), and a layer of 

 nerve-fibres (white matter). 



How do we account for the obliquity of the spinal nerves and for 



the cauda equina? 



When the spinal cord first appears it fills the entire spinal canal, 

 but at the time of birth the cord has apparently not grown so 

 rapidly as the vertebral column, for it then ends at the third 

 lumbar vertebra, and in the adult it ends at the first. Thus we 

 are able to explain the apparent origin of the spinal nerves above 

 their point of exit from the canal, and the increasing obliquity of 

 the nerves from above down, until finally, in the tuft of vertical 

 nerves below the extremity of the cord, we see the extreme degree 

 of this peculiarity. 



How do the spinal nerves develop? 



They are formed from cells arising from the epiblast lining the 



