Cold operates, in this capacity, solely through the sensation produced by its contact with the surface. Though a depressing agent in its direct action upon the part with which it comes in contact, it yet communicates to the nervous centres an impression, denominated the sensation or sense of cold, which is really excitant. This excitant impression may often be taken advantage of therapeutically to arouse the nervous system generally, or some great function, as the circulatory or respiratory, the nervous centres of which may be prostrated. Cold is applied in various methods, and in various degrees of intensity for this purpose.

1. One of the mildest is by simply sprinkling cold water on the face, or dashing it with a slight degree of force by means of the finger. This often answers an excellent purpose in obviating syncope or asphyxia, and will suffice to arouse a patient from slight attacks of this kind.

2. A stronger method, is to dash it upon the head, back of the neck, and shoulders, out of a small vessel. This plan may be resorted to in cases of stupefaction from narcotic poisoning, as by opium for example, in which it will sometimes temporarily excite the susceptibility of the brain, and enable it to feel the impression of emetic medicines upon the stomach. Vomiting may thus be induced, which the medicine unaided might be incompetent to effect. In asphyxia from hydrocyanic acid, it is one of the most efficient means of bringing about a return of respiration. One of the first effects of cold, suddenly applied to the surface, is to induce an involuntary inspiratory effort, through its excitant influence on the nervous centre of respiration. This inspiration is what is needed for the recommencement of the vital movements. Air is inhaled, the blood is changed, the pulmonary capillaries, before torpid, carry onward the arterialized fluid, this reaches the heart, and the central organ of circulation, thus reanimated, sends the life-giving current everywhere through the body. If the poison has ceased to act, this simple measure may be sufficient; if not, it at least gives the opportunity for the employment of others more efficient. So also in the asphyxia from chloroform, and other poisons operating in a similar manner. Spasmodic closure of the glottis in hysteria, epilepsy, and laryngismus stridulus may be relieved in the same way; the excitant impression on the nervous centre, which causes the effort at inspiration, operating so as to relax the spasm which would impede the entrance of air.

3. A third and still more energetic method of applying cold for the purpose of a nervous stimulant, is by means of the cold shower bath. This may be employed, in cases of mental torpor, to rouse the energies of the brain; but care must always be taken that there is sufficient energy in the system to insure reaction against the immediately depressing influence of the cold on the surface.

4. A fourth method, more energetic as a nervous stimulant than either of the others, is cold affusion, or the pouring of cold water, in considerable quantities, from pails, or other vessels, held two or three feet above the body. This was recommended by Dr. Currie, of Liverpool, in fevers, with the view of cutting them short; an object which is effected in some cases of typhus, and possibly in remittents, but never in typhoid or enteric fever. The operation of the remedy is partly by cooling the heat of the surface; but it is mainly by the powerful shock upon the nervous centres, breaking the morbid associations upon which the disease may depend, and introducing a new series of actions which may subside into health. But the measure does not always succeed, and is somewhat hazardous where it does not; and in general there are other and milder means which are in the end probably more effectual. But there is a condition of great danger, in which cold affusion has been employed with much asserted success; and in which, should other measures fail, the practitioner would be justified in having recourse to it. I allude to the condition of collapse sometimes occurring at the commencement of malignant febrile diseases, and often characterizing the onset of pernicious or congestive fever. The patient in this condition, though really cold both without and within, as shown by the state of the breath, not unfrequently complains of burning heat. The sensation of the cold affusion is said to be agreeable to him at first; and the moment at which he begins to feel an unpleasant sense of chilliness, is the time to cease with the application of cold, and to apply measures requisite for favouring reaction; as wiping him dry, wrapping him up in blankets, etc. The powerful stimulus of the cold to the nervous centres is, under these circumstances, the great agent of restoration. Other affections in which cold affusion has been used are the collapse of cholera, asphyxia from carbonic acid, and puerperal convulsions. In the use of affusion the patient should be stripped naked, and may either have the water poured upon him from above while sitting over a tub; or, what is more convenient, he may be placed horizontally, and the affusion be made from a pitcher from one end of the body to the other.

5. The most energetic method of applying cold as a nervous stimulant is by immersing a patient in the cold bath. Generally speaking, this is too powerful for the object aimed at, which can be sufficiently accomplished by one of the other plans. Nevertheless, I have known it to be resorted to in one case of apparently almost fatal prostration in the pernicious paroxysm, with the supposed effect of producing reaction, and saving the life of the patient.

The temperature of the water for these purposes may be from 33° to 60° F., but must vary with the method of application. When merely sprinkled upon the face, it can scarcely be too cold. When dashed moderately on the head or shoulders, or used in the form of a shower bath, it may be at the mean between the two extremes mentioned. When poured from a height over the whole body, or used in the form of a bath, it may be between the mean and the highest temperature. The period of application should always be very short. It is the shock that is wanted, not the depressing influence of the cold. If it be continued too long, the depressing effect will reach even the nervous centres themselves, and a result exactly the reverse of that desired will be obtained. An instant is often enough; and the application should seldom be protracted more than two or three minutes.