728 THE SPINAL REFLEXES. 



THE SPINAL REFLEXES. 



By reflex movements are understood such as are induced by irritation 

 of a centripetal (sensory) nerve. The latter takes up the irritation, and 

 conveys it to the spinal cord, the cellular gray matter of which acts as 

 the reflex center. Here the impulse is transferred to the motor centrif- 

 ugal path. Three factors are thus concerned in a reflex movement 

 and together constitute the so-called reflex arc: the centripetal fiber, 

 the transferring center in the gray matter, and the centrifugal fiber (Fig. 

 251, s h v m). The activity of the will is excluded in the occurrence of 

 a reflex movement. 



Three varieties of reflex movement are distinguished: i. The 

 simple or partial reflex, which is characterized by contraction of a single 

 muscle or at most of a small group of muscles as a result of stimulation of 

 a sensory nerve. Examples of this type of reflex movement are the 

 contraction of the quadriceps femoris muscle following a tap on the knee ; 

 and the closure of the lids as a result of irritation of the cerebral nerves 

 on the eye. 2. The widespread Uncoordinated reflex, or reflex spasm. 

 This occurs in the form of tonic or clonic contractions involving entire 

 groups of muscles, or even all of the muscles of the body. The reflex 

 spasm is due to a double cause : (a) the, gray matter of the spinal cord 

 may be in a condition of excessive irritability, so that the conveyed stim- 

 ulus can be readily transferred from its point of entrance to the readily 

 irritated adjacent central areas. Such excessive irritability is caused 

 "by certain poisons, particularly strychnin, and also brucin, caffein, atro- 

 pin, nicotin, carbolic acid, etc. The slightest touch of an individual pois- 

 oned by strychnin is sufficient at once to throw all of the muscles of the 

 body into spasm. Reducing the temperature of the body to 23 C. like- 

 wise gives rise in the dog to marked reflex irritability. Also certain 

 pathological and morbid conditions may bring about a similar result. An 

 illustration is the excessive irritability in cases of hydrophobia and teta- 

 nus. Conversely, the central organs may be placed in a condition in 

 which extensive reflex convulsions cannot occur. Thus, in the state 

 of apnea the convulsions usually attending strychnin-poisoning do not 

 occur, in consequence of the passive artificial respiratory movements, 

 which cause stretching of the cutaneous nerves of the abdomen and chest. 

 Also the practice of other periodic passive movements of portions of the 

 body gives rise to a similar condition ; and considerable reduction in the 

 temperature of the spine inhibits reflex convulsions, (b) Extensive reflex 

 convulsions may, however, occur when the reflex stimulation is severe. 

 Examples of this kind are observed in man, as in the widespread con- 

 vulsions attending intense neuralgias. 



The general convulsion is extensor in type (involving the spinal column: 

 opisthotonus) , because the strength of the extensors is greater than that of 

 the flexors. Nerves arising from the medulla oblongata may be excited reflexly 

 also by stimulation of remotely situated central nerves, without the occurrence 

 of general convulsions. 



Strychnin, the most powerful of the poisons exciting reflex convulsions, acts 

 directly upon the ganglia of the gray matter of the spinal cord. Therefore, the 

 same reflex convulsions occur when the poison (in the frog, after ligation of the 

 heart) is applied directly to the exposed spinal cord. The spasms occur after 

 mechanical, thermal, or electrical stimulation, but not after chemical stimulation. 

 During the spasm the heart stops in diastole from irritation of the vagus, and 

 the pressure in the arteries undergoes a marked rise as a result of irritation of 



