X REFLEX ACTION 171 



tinuous 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.f) 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 

 (p. 167). 



The path of a nervous impulse 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 

 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 after passing through the cells of the dorsal 

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

 ducted by an axis-fibre process continuous with the neur- 

 axis 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 movements are of such a 

 nature as to withdraw the part stimulated from the irritating 

 substance. Moreover, as shown by the experiment of apply- 

 ing acid to various parts of the body, the movements are 



