MOTOR AND SENSORY NERVES. 67 



same source. These facts, which were fully appreciated by 

 the ancients, show that the general system of nerves is 

 endowed with motor and sensory properties, the question 

 being simply whether these be inherent in the same fibres 

 or belong to fibres physiologically distinct and derived from 

 different parts of the central system. This question, which 

 was solved only about half a century ago, will be the first 

 to engage our attention. 



Distinct Seat of the Motor and Sensory Properties of the 

 Spinal Nerves. All of the nerves that take their origin 

 from the spinal cord are endowed with motor and sensory 

 properties. These nerves supply the whole body, except 

 the head and other parts receiving branches from the cranial 

 nerves. They arise by thirty-one pairs from the sides of the 

 spinal cord, and each nerve has an anterior and a posterior 

 root. The anatomical differences between the two roots are 

 that the anterior is the smaller, and ha"s no ganglion. ' The 

 larger, posterior root presents a ganglionic enlargement in the 

 intervertebral foramen. Just beyond the ganglion, the two 

 roots coalesce and form a single trunk. The nerve-fibres in 

 the two roots are not of the same size, the anterior fibres 

 measuring on an average about one-fourth more than the 

 posterior fibres. 1 The structure of the ganglia of the poste- 

 rior roots has already been considered sufficiently in detail. 3 



It would be unprofitable to discuss the vague ideas of the 

 older anatomists and physiologists with regard to the proper- 

 ties of the roots of the spinal nerves, and we can date our in- 

 formation upon this point from the suggestion of Alexander 

 Walker, in 1809, that one of these roots was for sensation 

 alone and the other for motion. 3 It is most remarkable, 

 however, that "Walker, from purely theoretical considera- 



1 KOLLIKER, Elements d'histologie humaine, Paris, 1868, p. 339. 



2 See page 51. 



3 WALKER, New Anatomy and Physiology of the Brain in particular and of 

 the Nervous System in general Archives of Universal Science, Edinburgh, 1809, 

 vol. iii., pp. 173, 174. 



