386 THE NERVOUS SYSTEM. 



which it maybe reflected ; (3), one or more centrifugal nerve- 

 fibres, upon which this impression may be reflected, and by 

 which it may be conducted to the contracting tissue. In the 

 absence of any one of these three conditions, a proper reflex 

 movement could not take place ; and whenever impressions 

 made by external stimuli on sensitive nerves give rise to mo- 

 tions, these are never the result of the direct reaction of the 

 sensitive and motor fibres of the nerves on each other ; in all 

 such cases the impression is conveyed by the sensitive fibres to 

 a nervous centre, and is therein communicated to the motor 

 fibres. 



2. All reflex actions are essentially involuntary, and may 

 be accomplished independently of the will, though most of 

 them admit of being modified, controlled, or prevented by a 

 voluntary effort. 



3. Reflex actions performed in health have, for the most 

 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 illlustration 

 of the first point, may be mentioned movements of the diges- 

 tive 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 larynx. 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 

 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 with- 

 drawal of the stimulus. 



CEREBROSPINAL NERVOUS SYSTEM. 



The physiology of the cerebro-spinal nervous system in- 

 cludes that of the spinal cord, medulla oblongata, and brain, 

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



