490 THE NERVOUS SYSTEM. 



ments, and the contraction of the eyelids and the pupil to 

 exclude many rays of light, when the retina is exposed to 

 a bright glare. These and all other normal reflex acts 

 afford also examples of the mode in which the nervous 

 centres combine and arrange co-ordinately the actions of 

 the nerve-fibres, so that many muscles may act together for 

 the common end. Another instance of the same kind is 

 furnished by the spasmodic contractions of the glottis on 

 the contact of carbonic acid, or any foreign substance, with 

 the internal substance of the epiglottis or larynx. Examples 

 of the purposeless irregular nature of morbid reflex 

 actions are seen in the convulsive movements of epilepsy, 

 and in the spasms of tetanus and hydrophobia. 



4. Reflex muscular acts are often more sustained than 

 those produced by the direct stimulus of muscular nerves. 

 As Volkmann relates, the irritation of a muscular organ, 

 or its motor nerve, produces contraction lasting only so 

 long as the irritation continues ; but irritation applied to 

 a nervous centre through one of its centripetal nerves, may 

 excite reflex and harmonious contractions, which last some 

 time after the withdrawal of the stimulus. 



CEREBRO- SPINAL NERVOUS SYSTEM. 



The physiology of the cerebro-spinal nervous system 

 includes that of the spinal cord, medulla oblongata, and 

 brain, of the several nerves given off from each, and of the 

 ganglia on those nerves. It will be convenient to speak 

 first of the spinal cord and its nerves. 



Spinal Cord and its Nerves* 

 The spinal cord is a cylindriform column of nerve -sub- 



* Fig. 133. View of the cerebro-spinal axis of the nervous system 

 (after Bourgery). The right half of the cranium and trunk of the 

 body has been removed by a vertical section; the membranes of the 

 brain and spinal marrow have also been removed, and the roots and first 

 part of the fifth and ninth cranial, and of all the spinal nerves of the 

 right side, have been dissected out and laid separately on the wall of the 



