472 ELEMENTARY PHYSIOLOGY less. 



motor nerve, (3) to Clarke's column (Fig. 143), and thence 

 to the cerebellum along the direct cerebellar tract (p. 491). 



Structurally we may regard the nerve-fibres of the 

 posterior roots of the spinal cord as taking their origin 

 from one process of a cell in the spinal ganglion, in the 

 same way that the fibres of the anterior rot)t originate in 

 one process of a nerve-cell in the anterior horn of the grey 

 matter. This accounts for the peculiar way in which the 

 fibres of the posterior root make their connection with the 

 cord, and also for the most obvious function of the spinal 

 ganglia of which we shall speak presently. 



6. The Functions of the Roots of the Spinal 

 Nerves.— If the trunk of a spinal nerve be irritated in 

 any way (at x in Fig. 152), as by pinching, cutting, 

 galvanising, or applying a hot body, two things happen : 

 in the first place, all the muscles to which filaments of 

 this nerve are distributed contract ; in the second, pain is 

 felt, and the pain is referred to that part of the skin to 

 which fibres of the nerve are distributed. In other words, 

 the efi'ect of irritating the trunk of a nerve is the same as 

 that of irritating its component fibres at their terminations. 



The eff"ects just described will follow upon irritation of 

 any part of the branches of the nerve : except that when a 

 branch is irritated, the only muscles directly aff'ected, and 

 the only region of the skin to which pain is referred, will 

 be those to which that branch sends nerve-fibres. And 

 theee effects will follow upon irritation of any part of a 

 nerve from its smallest branches up to the point of its 

 trunk, at wiiich the anterior and posterior bundles of root 

 fibres unite. 



If the anterior bundle of root fibres be irritated in the 

 same way (at ;/, Fig. 152) only half the previous effects 

 are brought about. That is to say, all the muscles to 

 which the nerve is distributed contract, but no pain is felt. 



So again, if t\iQj)ufiterior, ganglionated bundle be irritated 

 (at 2, Fig. 152) only half the effects of irritating the 

 whole trunk is produced. But it is the other half ; that is 

 to say, none of the muscles to which the nerve is distributed 



