380 CONVULSIVE FORMS OF REFLEX ACTION. 



excited by the irritation which results from the pressure of 

 the tooth as it rises against the unyielding gum ( 174). 

 They are often occasioned, too, by the presence of indigestible 

 or injurious substances, or of intestinal worms, in the alimen- 

 tary canal ; and will cease as soon as this is properly cleared 

 out. Again, in Tetanus or "lockjaw" resulting from a lace- 

 rated wound, the irritation of the injured nerve is the first 

 cause of the convulsive action ; and a similar local irritation 

 is often the origin of Epileptic fits, in which the convulsion 

 is accompanied by loss of consciousness. When these com- 

 plaints prove fatal, it is usually by suffocation, the muscles 

 of respiration being fixed by the convulsive action, in such a 

 manner that air cannot pass either in or out. 



474. In other forms of convulsive disorders, however, the 

 cause of irritation may directly affect the Spinal Cord, instead 

 of being conveyed to it by the nerves from a distance. This 

 seems to be the case, for example, in Hydrophobia ; which 

 terrible complaint is probably due to a poison introduced into 

 the blood by the bite of the rabid animal, and conveyed by 

 the circulating current to the nervous centres. So, when the 

 poison termed Strychnia has found its way into the circula- 

 tion, the whole Spinal Cord is thrown into such an excitable 

 state, that the slightest stimulus produces the most violent 

 convulsive movements, which succeed one another in extra- 

 ordinary variety. And the teething-convulsions of infants 

 often depend more upon a peculiar excitable state of the 

 spinal cord, which results from atmospheric impurity, and is 

 removed by change of air, than they do upon the irritation 

 of the gums. By knowing, as he now does, the part of the 

 nervous system on which these convulsive disorders depend, 

 the Physician is enabled to apply his remedies with much 

 greater precision than heretofore, and to form a much more 

 accurate estimate of the danger which attends them. 



Functions of the Ganglia of Special Sense. Consensual Actions. 



475. It has been seen that the nerves of special sense 

 those of smell, sight, and hearing terminate in ganglionic 

 centres peculiar to themselves, which are lodged within the 

 skull, and form part of what is commonly termed the brain, 

 though distinct both from the Cerebrum and the Cerebellum. 

 These Sensory Ganglia are almost the only representatives of 



