586 NERVOUS SYSTEM. 



there was free purgation, and in one there was " some slight muco-faecal dis- 

 charge." From these, Wood concluded that while section of the cervical 

 pneumogastrics, in the great majority of instances, arrests gastro-intestinal 

 secretion and prevents the action of purgatives upon the intestinal canal, a 

 few exceptional cases occur in which these effects are riot observed. 



It would be interesting to determine whether the pneumogastrics influ- 

 ence the intestinal secretions through their own fibres or through filaments 

 received from the sympathetic system ; but there are no experimental facts 

 sufficiently definite to admit of a positive answer to this question. If the 

 action take place through the sympathetic system, as in the case of the stom- 

 ach, the filaments of communication join the pneumogastrics high up in the 

 neck. 



The cranial nerves that have been considered in this chapter are the 

 third, fourth, fifth, sixth, seventh, tenth, eleventh and twelfth. The ana- 

 tomical and physiological history of the olfactory (first), optic (second), 

 auditory (eighth), gustatory (branch of the seventh and a part of the ninth) 

 and of the general sensory nerves, as far as they are concerned in the sense 

 of touch, belongs properly to the chapters on the special senses. 



CHAPTEE XVIII. 



THE SPINAL CORD. 



General arrangement of the cerebro-spinal axis Membranes of the encephalon and spinal cord Cephalo- 

 rachidian fluid Physiological anatomy of the spinal cord Columns of the Cord Direction of the 

 nerve-fibres in the cord General properties of the spinal cord Motor paths in the cord Sensory paths 

 in the cord Relations of the posterior white columns of the cord to muscular co-ordination Nerve-centres 

 in the spinal cord Reflex action of the spinal cord Exaggeration of reflex excitability by decapitation, 

 poisoning with strychnine etc. Reflex phenomena observed in the human subject. 



THE nervous matter contained in the cavity of the cranium and in the 

 spinal canal, exclusive of the roots of the cranial and spinal nerves, is known 

 as the cerebro-spinal axis. This portion of the nervous system is composed 

 of white and gray matter. The fibres of the white matter act solely as con- 

 ductors. The gray matter constitutes a chain of ganglia, which act as nerve- 

 centres, receiving impressions and generating the so-called nerve-force. Cer- 

 tain parts of the gray matter also serve as conductors. 



The cerebro-spinal axis is enveloped in membranes, which are for its pro- 

 tection and for the support of its nutrient vessels. It is surrounded to a cer- 

 tain extent with liquid, and it presents cavities, as the ventricles of the brain 

 and the central canal of the cord, which contain liquid. The gray matter 

 is distinct from the white, even to the naked eye. In the spinal cord the 

 white substance is external and the gray is internal. The surface of the 

 brain presents an external layer of gray matter, the white substance being 



