Nervous Tissue 



43 



degree, but more uniformly, forming the spinal cord, with a small central 

 canal. 



As the medullary tube forms, some cells pass outwardly from the pos- 

 terior portion to form the spinal ganglia. 8 At an early period, not defi- 

 nitely determined, other cells migrate from the growing spinal cord (or 

 possibly from the spinal ganglia) , to form the sympathetic ganglia, and 

 other visceral ganglia, more remote from the spinal ganglia. From the 

 anterior part of the medullary tube there is a migration of those cells 



CEPHALIC 



fLEXURE 



Fig. 28. Profile view of the brain of a human embryo of six weeks. (Modified 

 from Cunningham, after His.) A, myelencephalon. B, metencephalon. C, isthmus. 

 D, mesencephalon. E, diencephalon. F, telencephalon. 



which become the neurons in the retina of the eye and those which become 

 the ganglion cells in the cochlea of the ear. Possibly also the olfactory 

 receptors originate in this way. 9 



From the brain, spinal cord, spinal ganglia, and visceral ganglia, the 

 nerve cells send processes (nerve fibers) to the various tissues of the 



8 For the significance of the term ganglion see page 49. 



* It has been a question whether the four autonomic or visceral ganglia in the head 

 are formed by the migration of cells from the anterior part of the medullary tube, 

 or from the sympathetic ganglia of the trunk. Recent investigations point to forma- 

 tion from both sources. 



