86 HAND-BOOK OF PHYSIOLOGY. 



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

 admit of being modified, controlled, or prevented by a voluntary effort. 



(c) Keflex 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 pur- 

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

 ments of the digestive canal, the respiratory movements, and the con- 

 traction 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 sur- 

 face 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. 



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

 by the direct stimulus 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 contrac- 

 tions, which last some time after the withdrawal of the stimulus (Volk- 

 mann). 



Classification of Reflex Actions. Reflex actions maybe classified 

 as follows (Kuss): 1. Those in which both the centripetal and cen- 

 trifugal nerves concerned are cerebro-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 convulsions pro- 

 duced by intestinal worms, or hysterical convulsions. 4. Those in which 

 both centripetal and centrifugal nerves are of the sympathetic system: as, 

 for example, the obscure actions which preside over the secretion of the 

 intestinal fluids, those which unite the various generative functions and 

 many pathological phenomena. 



Relations between the Stimulus aud the Resulting" Reflex 

 Action. Certain rules showing the relation between the resulting reflex 

 action and the stimulus have been drawn up by Pflliger, as follows: 



1. Laiv of unilateral reflection. A slight irritation of sensory nerves 

 is reflected along the motor nerves of the same region. Thus, if the skin 

 of a frog's foot be tickled on the right side, the right leg is drawn up. 



