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, i 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 soo 
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 
