502 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



sciousness is sufficiently on the alert to allow sensations to guide, direct, 

 and control the motions of the liinbs, their actions appear to be 

 performed without the agency of the will, which may be entirely con- 

 centrated upon some interior mental operation. It is certain that, in 

 Birds, the movements of flight may be performed after the removal of 

 the Cerebrum. 



885. There are many irregular or abnormal reflex actions, performed 

 through the instrumentality of the Spinal Cord, the study of which is of 

 the highest importance to the Medical Man. It is probable that all Con- 

 vulsive movements are produced through its agency and that of the 

 Medulla Oblongata ; for it has been found, by repeated experiments, 

 that these movements are never produced by injuries of the Cerebral 

 hemispheres. Convulsive movements may be of three kinds. 1. They 

 may be simply reflex ; being the natural result of some extraordinary 

 irritation. 2, They may be simply centric ; depending upon a peculiar 

 condition of the ganglionic centre of the Spinal Cord, which occasions 

 muscular movements without any stimulation. This may be dependent 

 upon an abnormal state of the Blood. We know that it may be pro- 

 duced by the introduction of certain poisons (as strychnia) into the cir- 

 culation ; and it is probable that morbid matters generated within the 

 body may have the same effect. 3. They may depend upon the com- 

 bined action of both principles ; the nervous centres being in a very 

 irritable state, which causes very slight irritations (such as would other- 

 wise be inoperative) to excite violent reflex or convulsive movements. 

 This last is by far the most common cause of the convulsive actions, 

 that occur in various diseased conditions of the system. Thus, convul- 

 sions are not unfrequent in children, during the period of teething; 

 being produced by the irritation, which results from the pressure of the 

 tooth, as it rises against the unyielding gum. In this case, the stimulus 

 would scarcely be sufficient to produce the violent result, were it not 

 for a peculiarly excitable state of the Spinal Cord, brought about by 

 various causes. In like manner, when such an excitable state exists, 

 convulsions may be occasioned by the presence of intestinal worms, of 

 irritating substances, or even simply of undigested matters, in the ali- 

 mentary canal ; and will cease as soon as they are cleared out, in the 

 same manner as the convulsions of teething may often be at once 

 checked, by the free lancing of the gums. 



886. The influence of the condition of the Spinal Cord itself, is 

 peculiarly seen in the convulsive diseases termed Hydrophobia, Teta- 

 nus, Epilepsy, and Hysteria. In the first of these, not only the Spinal 

 Cord, but the Medulla Oblongata, and the ganglia of Special Sense, 

 are involved ; their peculiar condition being the result, it would appear, 

 of the introduction of a poison into the blood. It is most remarkable 

 that the Cerebrum should so completely escape its influence. When 

 the state of intense excitability in these centres, is once established, 

 the slightest stimulus is sufficient to bring about convulsive movements 

 of the^ utmost violence. It is characteristic of this complaint, that the 

 stimuli most effectual in exciting the movements, are those which act 

 through the ^ nerves of special sense; thus the sight or the sound of 

 water will bring on the paroxysm ; and any attempt to taste it increases 



