468 HANDBOOK OF PHYSIOLOGY. 



the effect is manifested. In the absence of any one of these conditions, 

 a proper reflex action cannot take place; and whenever, for example, 

 impressions made by external stimuli on sensory nerves give rise to 

 movements, these are never the result @f the direct reaction of the sen- 

 sory and motor fibres of the nerves on each other; in all such cases the 

 impression is conveyed by the afferent fibres to a nerve-centre, and is 

 therein communicated to the motor fibres. 



(b) All reflex actions are essentially involuntary, though most of 

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

 effort. 



(c) Reflex actions performed in health have, for the most part, a dis- 

 tinct 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 

 movements 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 nor- 

 mal 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 gas, or any foreign substance, 

 with the surface of the epiglottis or larynx. Examples of the purpose- 

 less irregular nature of morbid reflex action are seen in the convulsive 

 movements of epilepsy, and in the spasms of tetanus and hydrophobia. 



(d) Reflex muscular acts are often more sustained than those produced 

 by the direct slimulus of muscular nerves. 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. 



Classification of Reflex Actions. Reflex actions may be classified 

 as follows: 1. Those in which both the centripetal and centrifugal 

 nerves concerned are cere bro-spinal; e.g., deglutition, sneezing, coughing, 

 and, in pathological conditions, tetanus, epilepsy. 2. Those in which 

 the centripetal nerve is cerebro-spinal, and the centrifugal is sympathetic, 

 most often vaso-motor; e.g., secretion of saliva, or gastric juice: blushing 

 or pallor of the skin. 3. Those in which the centripetal nerve is of the 

 sympathetic system, and the centrifugal is cerebro-spinal. The majority 

 of these are pathological, as in the case of convulsion, produced by intes- 

 tinal worms, or hysterical convulsions. 4. Those in which both centrip- 

 etal and centrifugal nerves are of the sympathetic system: as, for exam- 

 ple, in the nervous mechanism concerned in the secretion of the intestinal 



