X REFLEX ACTION 171 



being lost, passes into the ventral horn of the grey 

 matter, its neuraxis being directly continuous with the 

 axis-fibre process of one of the large motor nerve-cells 

 (v. c) : the remaining processes of these cells simply 

 branch out in the neuroglia. The sensory fibres (s. /) 

 are traceable into the dorsal root (d. r) ', in passing 

 through the ganglion of the root (g) they are found to 

 be continuous with its simple ("bipolar") nerve-cells 

 (g. c), and then pass into the cord. Instead, however, 

 of entering the grey matter at once, they pass forwards 

 as well as backwards for some distance, along the white 

 matter of the cord, giving off numerous branches, or 

 collaterals (col), which, losing their medullary sheaths, 

 enter the dorsal horn of the grey matter and branch out 

 into a complex series of fine fibres which interlace with 

 the similar arborisations of the nerve-cells in the cord 

 (p. 167). 



The path of a nervous impulse known as the reflex arc 

 or chain will now be obvious. The stimulus applied to 

 the skin (Fig. 55, S) is conducted by a sensory fibre to 

 the nerve-trunk and by the dorsal root to a cell in 

 the ganglion and thence to the spinal cord ; it then 

 passes along the white matter of the cord, enters the 

 grey matter, and is conducted by the collaterals to the 

 nerve-cells of the ventral horn, either directly, or in- 

 directly after passing through the cells of the dorsal 

 horn : from one of the cells of the ventral horn it is con- 

 ducted by an axon which becomes the neuraxis of a 

 nerve-fibre, which, leaving the cord by a ventral root, 

 passes along the nerve-trunk and finally goes to a muscle 

 (M) as a motor fibre. 



It will be noticed that a single stimulus applied to 

 the skin may result in the contraction of numerous 

 muscles as, e.g., when the application of a drop of 

 acid to the toe causes the lifting of the leg and that the 



