It is supposed by some that diffusible Stimulants act simply on the ganglionic system of nerves, being able through them to excite the functions of the heart, vessels, and glands. But Ammonia is used with advantage in some cases in which the whole nervous system is implicated in the disorder. It has been employed in cases of Typhus, of simple febrile exhaustion, of Hysteria, Epilepsy. In such affections the brain is at least involved, if not often the chief seat of the disorder. Spasms and convulsions are due to a general derangement of the nervous forces, and particularly of the brain, in which these centre and originate. Stimulants, by exalting the natural functions, counteract and control this derangement. It is thus that Ammonia and volatile oils are of use in convulsive disorders; but they are seldom of any permanent efficacy, as in most cases of Hysteria and Epilepsy there is at the bottom a blood-disease, of which the spasmodic fits are only the symptom and outbreak. (_Vide_ p. 176.) So also is there a limit to the efficiency of Stimulants in such cases as Typhus fever; there is a certain point of exhaustion beyond which they are of no avail. For I have attempted to show that though Stimulants are competent to exalt nervous force, they are unable to supply vital force. A degree of nervous force is necessary to the continuance of life; but when the powers of life themselves are ebbing away, it is impossible then to prolong any further the thread of existence by the exhibition of stimulant medicines. The action of Ammonia is directly the reverse of that of Prussic acid, which is a General Sedative, producing Convulsions and Syncope. Ammonia is thus used in cases of poisoning by that liquid. (_Vide_ pp. 126, 227, 278, 283, 301.). STRYCHNIA. Class II. Div. I. Ord. II. STIMULANTIA SPECIFICA. Strychnia is the chief alkaloid and active principle of Nux Vomica. This medicine is comparatively simple in its action. It has no claim to the title of Sedative, which is sometimes applied to it. It does not affect the nerves generally. It has no direct action on the brain, but stimulates chiefly the function of the spinal cord, and its system of nerves. Of these nerves it affects the motor considerably more than the sensory branches. Along with the spinal cord, it doubtless also acts upon that part of the brain which is immediately associated with the spinal system of nerves. But the function of reflex motion, which is thought to reside in the spinal cord, is particularly stimulated. Strychnia is thus an exciter of muscular contraction and of motion; exalting sensation in a less degree. It has also apparently some action on part of the ganglionic system of nerves, by which it is enabled to promote the function of the stomach, and becomes temporarily a tonic when given in relaxed conditions of that organ. But it does not excite the action of the heart.