BY A. JEFFERIS TURNER, M.D. 81 



tiated into peripheral sense organs and central cells, receiving 

 nervous impressions from these sense organs. However this 

 may be, you will observe in a very early stage of the embryo of 

 a hen's egg, or of any other vertebrate, the appearance of a 

 superficial groove, bounded by two ridges of thickened cells. 

 These ridges increase in height, meet above, and coalesce, 

 forming a tube lined by cells which originated from those 

 covering the surface of the embryo, but have become distinct 

 from them. The forepart of this primitive nervous tube under- 

 goes very complicated changes, into which I will not enter, to 

 form the brain. The hinder portion retains to the end very 

 much of its primitive form, and constitutes the spinal cord of 

 the adult. The first step towards the connection of the 

 embryonic spinal cord with the other organs and tissues is a 

 budding out of groups of cells along its dorsal surface on each 

 side. The cell-buds become detached as little cell-islands, which 

 develop into the spinal ganglia. In the next place the cells of 

 these embryonic ganglia grow out into processes at each end, the 

 two processes of each cell travelling in opposite directions. The 

 centrally growing processes return to the spinal cord, and so 

 resume connection with the central nervous system. The 

 remaining processes have a peripheral direction, and form the 

 sensory nerve fibres. They are joined by outgrowths from the 

 anterior cells of the spinal cord, which grow out to form the 

 motor nerve fibres. At each vertebral segment a nerve is formed 

 by the union of one of the motor and sensory roots. I wish you 

 to try and picture to yourselves the peripheral growth of these 

 nerve fibres, how they insinuate themselves among the other 

 tissues, as the roots of a plant insinuate themselves between the 

 particles of the earth on which it grows. But the process is not 

 an aimless one ; each nerve cord, each branch, each filament takes 

 its determined course, and no part of the body is free from their 

 invasion. The sensory filaments form a network all over the 

 body, but of especial fineness on its surface. The motor 

 filaments seek out the developing muscles, and each one attaches 

 itself to its appropriate muscular fibre. If you try to realise this 

 you will gain a faint conception of the method by which one 

 strand is woven in the wonderful fabric of flesh common to all 

 of us. 



The purpose of this brief sketch has been to bring home to 

 your minds the real and great difference between the phenomena 



