38. The cases related in the Lectures on wounds of arteries, of mortification taking place in the foot and leg, after the division of the principal artery in the thigh, show that the maintenance of the life of a part depends on the blood. The cases now related show that neither an injury nor the division of the principal nerve, nor, perhaps, of all the nerves going to a part, will destroy that life. The complete failure of the circulation, in a part such as the foot, impairs, but does not totally destroy, the sensibility imparted by the nerves, until after the loss of life has taken place, or until decomposition is about to occur. An injury then to the nerve causes great pain, not usually at the part injured, but in the extreme parts supplied by it; some loss of the power of motion; some deprivation of its ordinary sensibility, as shown by a feeling of numbness, and an incapability, to a certain extent, of resisting heat or cold. When all the nerves have been divided, the power of moving the limb is lost, as well as its sensibility in a general sense. The temperature remains at a natural standard under ordinary circumstances, but no extra evolution of heat can take place by which cold is resisted, nor any absorption of it, which perhaps renders the application of a high temperature, particularly when combined with moisture, dangerous. The circulation is capable of maintaining the ordinary heat of a part, although it is deprived of the influence of the special nerves of sensation and of motion; but a greater evolution of heat appears to depend on something communicated by the nerves in a state of integrity. In the case of Sir P. Broke, this something appeared to be derived from the brain, on which part the wound was inflicted, and the transmission of which was interrupted by the injury. The evolution of animal heat has of late been supposed to be dependent on electricity, from the resemblance which exists between it and the nervous power, although the attempts to identify them have not been successful. That the evolution of heat is the result of nervous power, appears to be indisputable; in what that power consists, physiologists have yet to ascertain.
39. The best means of mitigating the pain, independently of the application of warmth--and cold rarely does good, as the sufferer soon finds out--is by the application of stimulants to the whole of the extremity affected, followed by narcotics. The tinctures of iodine and lytta, the oleum terebinthinæ, the oleum tiglii or cajeputi, the liquor ammoniæ or veratria, may be used in the form of an embrocation, of such strength as to cause some irritation on the skin, short, however, of producing any serious eruption. After the parts have been well rubbed, opium, belladonna, or henbane may be applied in the form of ointment; or the tincture of opium, henbane, or aconite may in turn be applied on linen. Great advantage has been derived in many neuralgic pains from the application of an ointment of _aconitine_, carefully prepared, in the proportion of one grain to a drachm of lard, at which strength it will sometimes irritate almost to vesication, as well as allay pain.