These may often be so employed as to become powerful means in the cure of various functional disorder. They operate either by a direct excitation of depressed nervous centres, or by revulsively relieving those which are in a state of irritation. There are two sets of them, one acting through the special senses, the other through the general sensibility.

1. Through the Special Senses

A sudden impression on the retina, as from a flash of sunlight in a dark place, or of lightning in the night, may sometimes rouse a torpid brain, when only functionally affected.

Similar Impressions on the sense of smell will often check asphyxia; and the disagreeable odour of fetid substances is one of the means by which they relieve nervous disorder.

A sudden sharp sound may arrest a threatened attack of syncope; and blowing in the ear is one of the remedies recommended for laryngismus stridulus. It is probable that music operates remedially, in part, by a moderately stimulant influence on the auditory centres; although something higher than sensation is here also concerned; for sounds arranged melodiously, or in harmony, have associations with our mental constitution, which often powerfully excite emotion, and thus produce nervous stimulation by an additional influence. In calming morbid nervous irregularity or excitement, music is well known to have had extraordinary influence from the times of the Hebrews, when the harp of David composed the mental distemperature of the Jewish monarch. I was once witness to a striking case of this kind. It was in a stage-coach full of passengers, among whom were two or three women who could sing. A young man was suddenly seized with symptoms of insanity. After having attempted to escape by jumping out of the window of the vehicle, he was seized by the passengers, and compelled to remain. The idea took possession of his mind that he was in the power of robbers, who intended to kill him. He began to scream violently and incessantly; and no assurances, nor anything else that could be done, seemed to pacify him. At length our female companions suggested the effects of music. The suggestion was approved, and they began to sing in concert; entertaining us with song after song with the greatest zeal and good nature. The poor maniac gradually began to show the influence of the music. His cries became less violent, and by degrees subsided into complete calmness; and before the strains ceased he was fast asleep The music probably acted as a stimulant, calming the perturbation, by bringing the nervous centres to a similar level of excitement, after which they fell into a state of exhaustion consequent on the previous exaltation, and sleep ensued.

There is no doubt that an excessively disagreeable taste may operate similarly in rousing the nervous centres of respiration and circulation.

A strong pressure on the upper lip will sometimes enable one to control an otherwise irresistible tendency to sneeze; and a smart slap on the back may surprise nervous spasm of the glottis into relaxation.

2. Through the General Sensibility

Smart pain produced by any cause will often relieve nervous disease. Not unfrequently a restless patient may be made to sleep by a pair of blisters to his extremities. I knew a gentleman who was never free from morbid hypochondriacal sensations except when he had a blister drawing upon his epigastrium. The pain produced by a sinapism over the stomach is probably quite as efficient as the revulsive effect of the inflammation, and even more so, in relieving spasm of that organ. But pain may be carried so far as to overwhelm instead of rousing the nervous centres, as we see constantly in violent spasms of the stomach, bowels, ureters, etc. As a remedial measure, therefore, it must be used with discretion. To this category belong the shock produced on the nervous centres by cold and by electricity, each of which merits a brief notice.