488 THE XERVOUS SYSTEM. 



part, a distinct purpose, and are adapted to secure some 

 end desirable for the well-being of the body; but, in 

 disease, many of them are irregular and purposeless. As 

 an illustration of the first point, may be mentioned move- 

 ments of the digestive canal, the respiratory movements, 

 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 larnyx. Examples of the 

 purposeless irregular nature of morbid reflex action 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 Yolkmann 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 



* Fig. 140. View of the cerebro-spinal axis of the nervous system 

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

 body lias 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 



