The first effect of the application of a cold body to the surface is the sensation of cold. This is owing to an impression conveyed from the point of application to the cerebral centres, and is really excitant to these centres. it is proportionate to the extent of the surface in contact with the cold body, and to the intensity of the cold. it may be disagreeable, and even severely painful, and sometimes serves to rouse the brain from a torpor into which it may have been thrown by some depressing influence. it was no doubt intended to guard against the evil effects of the cold by exciting attention, and, at the same time, contributing to reaction. But, while the nervous centres are thus stimulated, the part itself suffers a pure depression. The temperature, the circulation, and the special sensibility are all diminished; the surface becomes pale; the tissues shrink; and the peculiar function, whatever it may be, is impaired or suppressed. Perspiration is checked in the skin, the muscles are rendered stiff and unable to contract efficiently, nutrition suffers, and the part feels benumbed, and cannot duly appreciate the contact of bodies. it is true that there is pain in the part, and that a blow upon it is often exquisitely painful; but this is owing to the impression upon the distant nervous centres, and is merely referred by the mind to the part as its apparent seat. Should the cold be removed, reaction takes place; and, even during its application, if it be moderate, a similar tendency is often observable; but, with the continuance of the cold, especially if severe, this soon disappears; and even the capacity of receiving and transmitting impressions to the nervous centres ceases. A sedative effect is produced upon every constituent of the tissue, upon the blood-vessels, nerves, absorbents, and ultimate cells or molecules. There is in it less blood and less nervous action. Nor is it only the part chilled that suffers depression.

Other parts of the surface appear to sympathize, to a certain extent, with that which receives the first impression. That this effect is not owing simply to the reduction of the temperature of the blood, by the mingling with the whole circulating mass of that portion which passes through the vessels directly affected, is proved by the unaltered temperature of other portions of the body. MM. Tholozan and Brown-Sequard showed, by experiment, that the application of cold to one hand is followed by reduction of temperature in the other hand, without any appreciable diminution of that of the body generally. {Archives Generates, Nov. 1858, p. 613.) A similar sympathetic effect is probably transmitted to the interior mucous surfaces. At least this may be inferred from the efficiency of cold, externally applied, in arresting hemorrhage from these surfaces. The blood and nervous force, diminished in the skin and mucous membrane, must be concentrated in the great trunks and the parenchyma of the organs. Cold applied to the interior, to the stomach for example, has the same effects; and in this case also the sympathy between the outer and inner surfaces is exhibited; for the skin becomes cold, pale, and shrunken, when the impression upon the stomach is strong, and the tendency to reaction is surmounted. Every one is familiar with the great prostration, attended not unfrequently with gastric pains, which follows the drinking largely of very cold water, especially when the surface is perspiring, and the strength so far exhausted by exertion as to interfere with reaction.

The great discovery by Bernard of the contractile influence exercised by the sympathetic nervous centres, offers the means of a satisfactory explanation of the action of cold in producing contraction at once of the exterior and interior surfaces of the body. it is highly probable that the shock of cold, to whatever part applied, exercises a special excitant influence on the sympathetic centres, causing them to increase contraction of the capillaries, universal when the application of cold is general, but more or less limited when the localization of the cold determines the excitation only of certain ganglia, having special relation with the part refrigerated.

If the cold is sufficiently intense, the actions of the part are depressed until they cease altogether. Life, however, is not necessarily lost, even though the part may have been frozen; for, if care is taken to restore the natural temperature gradually, it resumes its functions, and its previous condition. if heat is too suddenly applied, the reaction is violent, inflammation takes place, and gangrene is apt to result. But, should the cold be continued sufficiently long, death will take place, in the part at least, so that, upon the reapplication of warmth, circulation within it is not resumed, and it is separated by sloughing.

When the cold is applied so as to affect the whole body, the depression is general, after the first efforts at reaction are past. Every reader is familiar with the effects of intensely cold weather on persons exposed to it without sufficient protection. At first the sensations are severely painful, and the cerebral centres are excited to resist the depressing influence. But, after a time, the sedative power of the cold seems to penetrate the interior of the body, the blood becomes universally chilled, and the nervous centres themselves lose their faculty of resistance. Languor, heaviness, and an irresistible disposition to inaction come on, drowsiness follows, and, if this is yielded to, the patient speedily sinks into a stupor from which he never awakens.

The effects of immersion in cold water are of the same general character, being more rapid and intense in proportion as the temperature of the water approaches the freezing point. The immediate phenomena are the sensation of cold, paleness of the surface, and universal shrinking of the body. When the temperature is but slightly reduced, the feeling is not altogether disagreeable, and reaction soon takes place, with a pleasant sensation of warmth. By a continuance, however, this ceases; and the subsequent influence is sedative. if the temperature of the bath approaches the freezing point, the sensation produced is intense, amounting to a violent shock upon the nervous centres, with general horripilation, shivering, gasping, or a kind of convulsive inspiration, and not un-frequently tumultuous action of the heart; but the efforts at reaction soon cease, and great depression follows. The skin is pallid or purple in patches, and greatly shrunken; the pulse gradually sinks until it can be felt no longer; severe pains are felt in the head, trunk, and limbs; the muscles and joints become stiff and inflexible; cramps in the extremities are not unfrequently experienced; and at length drowsiness comes on, speedily ending in a fatal stupor.

It must be obvious that an agency so powerful, and capable of producing effects so strongly indicated in many conditions of disease, must be susceptible of very useful therapeutic application. it is not less so that, if used inappropriately, or in excess, it may be productive of vast injury. Hence the danger of an instrument of this kind in uninstructed hands, to which, unhappily, it is too often entrusted.

Since Bernard's discovery before referred to the field for the therapeutic application of cold has been much extended, under the impression that it may be so applied externally, either alone or in connection with heat, as to bear with special force on certain nervous centres, especially the sympathetic ganglia, and thus produce, with considerable certainty, modifications in the condition of the parts under the influence of those centres such as may be indicated in disease. Dr. John Chapman, of London, has thus applied cold to an extensive series of diseases; and our countryman, Dr. E. M. Blanchard, of Connecticut, is scarcely behind him in the influence he ascribes to the remedy. (Boston Med. and Surg. Journ., April, 1864, p. 229.) But, notwithstanding the beauty of the hypothesis, it is a matter of great doubt whether we can give such a desirable direction to the agency of cold externally applied; and there must always, it seems to me, be considerable risk of bringing into action nervous centres other than those we may especially wish to influence. Thus, in the application of cold water or ice-bags to certain regions of the spine, with the object of influencing the sympathetic ganglia especially, how can we be assured, that the spinal centres of reflex action, and the nervous cords which in the spinal marrow connect all parts of the body with the brain, shall not equally feel the influence of the measure, and thus very much compromise the results?