Because, as the warmth expands the cuticle, and the blood begins to flow more freely through the vessels, _the nerves are made conscious of the movements of the blood_, and continue to be so until the circulation is equally restored to all the parts. [Verse: "In the sweat of thy face shalt thou eat bread, till thou return to the ground; for out of it thou wast taken: for dust thou art, and unto dust shalt thou return."--GENESIS III.] 1008. _Why do persons whose legs and arms have been amputated fancy they feel the toes or fingers of the amputated limb?_ Because the nervous trunk which formerly conveyed impressions from those extremities remains in the part of the limb attached to the body. _The mind has been accustomed to refer the impulses received through that nervous trunk to the extremity where the sensations arose._ And now that the nerve has been cut, the painful sensation caused thereby is referred to the extremity which the nerve supplied, and the sufferers for a time appear to _continue to feel the part which they have lost_. CHAPTER LI. 1009. _Why do we perspire?_ Because the skin is filled with very _minute pores_, which act as outlets for a portion of the water of the blood, that serves to _moisten and cool_ the surface of the body, and to carry away some of the matter _no longer needed in the system_. 1010. _How is the perspiration formed?_ By very small _glands_, which lie embedded in the skin. It is estimated that there are about 2,700,000 perspiratory glands distributed over the surface of the body, and that these glands find outlets for their secretion through no less than _seven millions of pores_. 1011. _What is insensible perspiration?_ Insensible perspiration is that _transmission of watery particles through the skin_ which is constantly going on, but which takes place so gently that it cannot be perceived. It is, however, very important in its results, as no less than _from twenty to thirty-three ounces of water may pass imperceptibly through the skin in twenty-four hours_. 1012. _What is sensible perspiration?_ Sensible perspiration is that moisture which exudes upon the skin _in drops large enough to be perceptible_, when the body is heated by exercise or other means. [Verse: "And Elisha sent a message unto him, saying, Go and wash in Jordan seven times, and thy flesh shall come again to thee, and thou shalt be clean."--II KINGS V.] 1013. _Why does a sudden change from heat to cold bring on illness?_ Because the effect of cold _arrests the action of the vessels of the skin_, and suddenly throws upon the internal organs the excretory labour which the skin should have sustained. 1014. _Why does a chill upon the skin frequently produce inflammation of the lungs?_