CHAP, i.] SENSORY NERVES. 485 



reality a purely motor nerve. So likewise is the hypoglossal, its 

 sensory fibres coming from the fifth, pneumogastric, and three 

 upper cervical nerves. The fifth is a mixed nerve entirely on the 

 plan of a spinal nerve, having distinct motor and sensory roots. 

 The glosso-pharyngeal seems to be essentially a sensory nerve, its 

 motor filaments springing from the fifth and facial nerves. Con- 

 cerning the vagus some have maintained that the pneumogastric 

 root proper is entirely sensory (afferent), and that all the efferent 

 functions of the vagus are dependent on the fibres of the spinal 

 accessory which join it. To this point we shall return when we 

 come to consider briefly the special functions of the several nerves. 



We have already stated (p. 106) that isolated pieces of motor 

 and of sensory nerves behave exactly alike as far as all the physical 

 manifestations attendant on the passage of a nervous impulse are 

 concerned ; the current of action makes its appearance in the same 

 way and seems to have the same characters in both kinds of 

 nerves. The same is also true, as far as we know, of nerves within 

 the body. 



Moreover, the rate at which nervous impulses travel appears to 

 be about the same in motor and sensory nerves ; at least we have 

 no evidence of any fundamental difference in this respect between 

 the two. We have seen that the velocity of a nervous impulse in 

 the motor-nerve of a frog is about 28 metres per sec. The velocity 

 of a motor impulse in man, as judged by the difference of the 

 latent period of the contraction of the thumb-muscles, when stimu- 

 lation is brought to bear on the motor-nerve at the wrist, or high 

 up in the arm, is about 33 metres per sec. In warm-blooded 

 animals, however, the rate of transmission of motor impulses is 

 very variable, being in particular closely dependent on temperature, 

 and probably also on other circumstances. Thus, it may range 

 from as low as 30 m. when the nerve is cooled to as high as 90 m. 

 when it is warmed. The velocity of a sensory impulse is estimated 

 by measuring the time taken between a stimulus being brought 

 to bear on some sentient surface, as the skin, and the making of a 

 signal by the individual experimented on at the instant that he 

 feels the stimulus. The time taken up in the sensory impulse 

 becoming converted into a sensation after reaching the nervous 

 central organs, in the mental operation of determining to make 

 the signal, and in the effort of making the signal, corresponds in a 

 way to the purely muscular portion of the latent period in the 

 experiment for determining the velocity of a motor impulse. The 

 application of the stimulus and the making of the signal (ex. gr. 

 closing a galvanic circuit) being both recorded on a rapidly 

 travelling surface, the time taken up in the whole operation 

 can be easily measured ; and the difference between the time taken 

 when the stimulus is applied to some spot separated from the 

 central nervous system by a short piece of nerve, ex. gr. the top of 

 the thigh, and that taken when a long piece of nerve intervenes, ex. 



