198 THE HUMAN BOD Y. 



per second in the motor nerves of a frog. In man it is 

 somewhat quicker, being 33 meters (108.24 feet) per second, 

 that is, about T ^- of the rate of the transmission of sound- 

 waves in air at zero. 



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

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

 afferent and eiferent 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 these too are mixed until very near their end- 

 ings, where the very finest terminal branches close to the 

 end organs, whether muscular fibres, secretory cells, or sen- 

 sory apparatuses, contain only afferent or efferent fibres. 

 If the nerve-trunk be one that arises from the spinal cord 

 and be examined progressively back to its origin, it will 

 still be found mixed, up to the point where its fibres sepa- 

 rate 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 told from examination of the dead 

 nerves since the best microscope fails to distinguish an 

 afferent from an efferent fibre, but is readily proved by 

 experiments first performed by Sir Charles Bell. If an 

 anterior root be cut and its outer end stimulated, the mus- 

 cles of the parts to which the trunk which it helps to form 

 is distributed, will be made to contract, and the skin will 

 be made to sweat also if the root happen to be one that 

 contains secretory fibres for the sweat-glands. On the other 

 hand, if the central end of the root (that part of it attached 

 to the cord) be stimulated no result will follow, showing that 

 the root contains no sensory, reflex, or excito-motor fibres. 

 With the posterior roots the reverse is the case: if one of 

 them be divided and its outer end stimulated, no observed 

 result follows, showing the absence of all efferent fibres; 



