_Observation._ When the inside of the arm or lower extremities rests upon a hard surface, the nerves may be compressed so as to deprive the parts of sensibility. This condition is called "numbness." 857. _The quantity of blood supplied to the skin modifies its sensitiveness._ If the quantity of blood is diminished, the sensibility of the skin will be impaired. This is demonstrated by noting the effects of cold upon the cutaneous tissue, the application of which contracts the blood-vessels, and drives the circulating fluid from this membrane, which is shown by the paleness, as well as by the shrivelled appearance of the skin. And, if this tissue is wounded while under the influence of cold, but little pain will be felt, and this chilling influence may be carried so far as not only to deprive the part of sensation, but of vitality. -=-=-=-=-=-=-=-=-=-=-=-= 854. Mention other conditions that affect these nerves. 855. What is the effect of the varying health or condition of the brain upon the sensitiveness of the skin? Give instances of this effect. 856. What is the result if a nervous trunk is divided or compressed? How may "numbness" in the limbs be produced? 857. Does the quantity of blood supplied to the skin affect its sensibility? -=-=-=-=-=-=-=-=-=-=-=-= 858. The influence of the blood upon the sensibility of the skin, is further demonstrated by the pain experienced when chilled extremities are suddenly exposed to heat. The nerves, by the sudden dilatation of the contracted blood-vessels, are put in vivid and rapid motion, which causes the painful and tingling sensation that we experience. In every part of the system, sudden changes produce unpleasant sensations, and frequently a diseased condition of the organs. _Observation._ When the hands, or other portions of the body, are frozen, or severely chilled, safety and comfort demand that circulation be restored to the parts by moderate exercise in a cool room. Not unfrequently, the vitality of the limb is destroyed by immersing it in hot water or holding it near the fire. 859. _The quality of the blood also influences sensation._ If the brain and other parts of the nervous system receive impure blood, their energy is depressed, and the sensibility of the skin rendered more or less obtuse. 860. _The condition of the cuticle modifies the impression made upon the cutaneous nerves._ 1st. When the cuticle has become thick and hard, like horn, as on the inside of the mason's hand, it enables him to ply his tools without much suffering, because the thickened cuticle diminishes the impressions made upon the nerves. -=-=-=-=-=-=-=-=-=-=-=-=