4 6 4 TEXT-BOOK OF PHYSIOLOGY. 



That the pyramidal tracts are the conductors of volitional 

 impulses throughout the length of the cord to its various seg- 

 ments has been made evident by the results of section, electric stimu- 

 lation, and disease. Division of the anterior and lateral columns 

 of one side of the cord in any part of its extent is invariably followed 

 by a loss of motion or paralysis of the muscles below the section, 

 while electric stimulation of the peripheral end of the isolated crossed 

 pyramidal tract is followed by marked characteristic movements of 

 the muscles. Similar results follow division of the pyramidal tract 

 in any part of its course from the cerebral cortex downward. Electric 

 stimulation of the cortical cells which give origin to the pyramidal 

 tract is also followed by contraction of the muscles of the opposite 

 side, while their destruction is attended by paralysis of the same 

 muscles. As the nutrition of the fibers is governed by the cells, it 

 follows that when the axon is separated from its cell it degenerates. 

 It has been found that a lesion of the pyramidal tract in any part of 

 its course is followed by descending degeneration, which is taken in 

 evidence that it conducts nerve impulses from above downward. 

 Thus experimental investigation and pathologic observation are in 

 accord in the view that physiologically these nerve-fibers are the 

 pathways for the transmission of motor or volitional impulses from 

 the encephalon to the spinal cord. 



Spino-encephalic or Sensor Conduction. The nerve impulses 

 that arise in consequence of impressions made on the terminals of 

 the nerves in the cutaneous and mucous surfaces, in the viscera and 

 in the muscles, are transmitted through the dorsal roots of the spinal 

 nerves to the cord. When transmitted through the cord to the cere- 

 bral hemispheres directly or indirectly, they are received by specialized 

 nerve-cells in the cortex and translated into conscious sensations. 

 The sensations thus arising may be divided into special and general 

 sensations. Of the former may be mentioned pain, touch, tem- 

 perature; of the latter may be mentioned hunger, thirst, fatigue, 

 well-being, etc. 



The pathways through the spinal cord that conduct these afferent 

 impulses to the brain are ill defined and imperfectly known, and only 

 for a few sensations can it be said that their pathways have been 

 determined. The reason for this obscurity lies partly in the diffi- 

 culties of experimentation, partly in the difficulties of interpretation. 

 Clinical observations are for special reasons more or less untrust- 

 worthy. 



Section of one lateral half of the cord, or a lesion involving the 

 one lateral half, as a rule abolishes all forms of cutaneous sensibility 



the 9pposite side below the injury. \ l This would seem to prove that 

 \ n 9Dl^[ m P ulses cross the median line of the cord immediately or 

 very shortly after entering. \ At the same 



