206 THE HUMAN BODY. 



lustrated in Fig. 62. The muscle M is dissected out with its 

 motor nerve attached, and the stimulus applied to the nerve 

 and not directly to the muscle. First the stimulus is given 

 to the nerve close to the muscle: it is then found that the 

 period of latent excitation, as shown by the greater length of 

 tu, is a very little longer than when the muscle is directly 

 stimulated. Next the stimulus is applied to the nerve, say two 

 inches from the muscle, and it is found that tu is consider- 

 ably longer, the increase in its length being due to the time 

 taken by the nervous impulse in travelling along two inches 

 of nerve. As we know the rate of movement of the surface 

 S, we can readily calculate the amount of the time increase. 

 The rate of travel of the nervous impulse as thus ascertained 

 is almost incomparably slower than that of an electric cur- 

 rent, being 28 metres (92.00 feet) per 1". In the motor nerves 

 of warm-blooded animals the rate of transmission is somewhat 

 faster. Considerable difficulties are met with in making cor- 

 responding measurements on afferent nerves, and the rates 

 obtained by different observers differ widely: probably the 

 impulse travels at about the same speed as in the motor nerves 

 of the same animal. 



Functions of the Spinal Nerve-Roots. The great ma- 

 jority of the larger nerve-trunks of the Body contain both 

 afferent and efferent nerve-fibres. If one be exposed in its 

 course and divided in a living animal, it will be found that 

 irritating its peripheral stump causes muscular contractions, 

 and pinching its central stump causes signs of sensation, 

 showing that the trunk contained both motor and sensory 

 fibres. If the trunk be followed away from the centre, as 

 it breaks up into smaller and smaller branches, it will be 

 found that the^e too are mixed until very near their endings, 

 where the very finest terminal branches close to the end- 

 organs, whether muscular fibres, secretory cells, or sensory 

 apparatuses, are only afferent or efferent. If the nerve- 

 trunk be one that arises from the spinal cord and be ex- 

 amined progressively back to its origin, it will still be found 

 mixed, up to the point where its fibres separate to enter 

 either a ventral or a dorsal nerve-root. Each of these latter, 

 however, is pure, all the efferent fibres leaving the cord by 

 the ventral or anterior roots, and all the afferent entering it by 

 the posterior or dorsal. This of course could not be learned 

 from examination of the dead nerves, since the best micro- 



