298 LONGITUDINAL TRANSMISSION IN THE CORD. 



The white or fibrous portion of the spinal cord is composed in part of 

 the spinal nerve fibres and in part of commissural ones. At one time it 

 was supposed that every one of the preceding continued uninterruptedly 

 to the brain. On this point, however, the weight of evidence will lead 

 us to infer that the vertical distance through which these fibres pass is 

 not very great, and that they are soon brought in connection with the 

 interior vesicular substance. If all the fibres passed uninterruptedly to 

 the brain, we should expect that the cord would increase in thickness by a 

 regular progression upward ; but this, as is shown in Fig. 138, is not the 

 case. Its enlargements correspond to the number of nerve roots given 

 off from the localities in which they occur. Thus, where many nerve 

 roots are required for the upper extremities, and again for the lower ones, 

 we notice such corresponding enlargements. The experiments of Volk- 

 mann show that these dilatations are as much owing to an increase of 

 the vesicular material as to an increased number of fibres. In the view 

 presented in the preceding chapter respecting nerve-arcs and the functions 

 of nerve-cells, we should be led to infer that every centrifugal and cen- 

 tripetal fibre of the cord is brought in connection with such a cell of the 

 gray material, and that it does not extend very far from its point of exit 

 or entrance. 



FUNCTIONS OF THE SPINAL CORD. The determination of the func- 

 Functions of tions of the roots of the spinal nerves by Bell has already 

 the spinal cord. k een referred to as one of the great discoveries of physiol- 

 ogy, and as furnishing a solid foundation for an exact knowledge of the 

 functions of the nervous system. The evidence of the truth of the doc- 

 trine that the anterior roots of these nerves are motor and the posterior 

 Bell's dis- sensory, is complete. Thus, if the anterior root of one of these 

 covery. nerves be divided, all those parts which are supplied by that 

 nerve will exhibit loss of motion, though their sensation is unimpaired ; 

 if the posterior root be divided, the sensibility of the parts is lost, though 

 the power of motion is unaffected. Similar evidence may also be ob- 

 tained by irritating the ends of the divided roots, muscular motion or 

 pain, as the case may be, being correspondingly observed. 



The spinal cord transmits impressions from the periphery to the brain, 

 Lon itudinai an< ^ converse ty enables the brain to bring into action the 

 transmission of motor nerves. Division of it at once causes an interruption 

 ifluences. Q vo i un t ai y mo tion and sensation in those parts supplied 

 by nerves below the place of the operation, the functions of the parts 

 above remaining unimpaired. But, though the influence of the brain in 

 exciting voluntary motion, and its capability of receiving sensations, is 

 thus cut off, the severed portion of the cord still possesses an automatic 

 power. 



This transmission of influences upward or downward is doubtless, to 



