575. On the other hand, whatever impedes the circulation and the interchange of the atoms of matter, diminishes animal heat. Common observation shows, that the extremities are not as warm when tight gloves or boots are worn as when they are loose. One reason is, the circulation of blood is impeded, which is attended with less frequent change of the particles of matter.

576. _The quant.i.ty of air which is inhaled modifies the heat of the system._ In the generation of heat in a stove, air, or oxygen, is as essential as the wood or coal. It is equally so in the production of animal heat. The oxygen of the inspired air should be in proportion to the carbon and hydrogen to be consumed. This requires voluminous lungs, together with free movements of the ribs and diaphragm. A person whose chest is small, and whose apparel is worn tight over the ribs, suffers more from the cold, and complains more frequently of chilliness and cold extremities, than the broad-chested and loosely dressed.

What do the experiments of Dr. Milne Edwards show? 574. Why does exercise influence animal heat? 575. What is the effect when the circulation of blood is impeded? Give examples. 576. Why do those persons that have broad chests and voluminous lungs suffer less from cold than the narrow-chested with small lungs?

_Observation._ Fishes that breathe by means of gills, as the cod, pike, &c., depend solely on the small quant.i.ty of oxygen that is contained in the air mixed with the water. Their temperature is not much greater than the medium in which they live. Whales, dolphins, &c., breathe by means of lungs, and the inhalation of atmospheric air makes their temperature about 100, independent of the heat of the element in which they live.

577. _The quality of respired air influences the generation of animal heat._ In vestries, and other public rooms, when crowded with an audience, where the ventilation is inadequate, the lamps will emit but a faint light, because the oxygen is soon expended, and there is not enough of the vivifying principle to unite with the oil and disengage light. In the human body, when the respired air has lost some of its life-giving properties, the combustion that takes place in different parts of the system is not so complete as when it contains a proper proportion of oxygen; and hence less heat is disengaged. For this reason, those persons that breathe impure air, either in the daytime or night, require more clothing, than those that work and sleep in well-ventilated rooms.

578. _The condition of the brain and nervous system affects the generation of animal heat._ If the brain is diseased, or the mind is absorbed in thought, depressed by sorrow, or aroused from fear, the breathing becomes slow and scarcely perceptible, and a chilliness pervades the body, particularly the extremities; while, on the contrary, if the mind and nervous system are excited by joyous and agreeable emotions, the circulation of blood is quicker, and the system more powerfully resists external cold. During sleep, when the brain is partially inactive, less heat is generated than when awake.

What is said of those fishes that breathe by means of gills? Of those that breathe by means of lungs? 577. Why do lamps give but a faint light in crowded, unventilated rooms? What effect on animal heat has impure air? 578. Mention the effects of some of the mental emotions on animal heat.

_Observation._ The preceding remark explains why an individual who sleeps in the same clothing that was adequate to prevent chills while awake, contracts a cold, unless he throws over him an additional covering.

579. _The state of the skin exercises much influence in the generation of heat._ If the functions of this membrane are not interrupted, more heat will be generated than when it is pallid and inactive. The action of the capillaries is most energetic when the skin is clean; on this account, before taking a walk or a ride, in cold weather, remove all impurities from the skin, by thorough ablution and vigorous friction.

580. _The amount and kind of clothing modify the temperature of the system._ Those persons that are well clothed have greater power to resist cold than the thinly apparelled, because both the evaporation and the radiation from the skin are impeded, and less heat, in consequence, is abstracted from the body. If the articles of apparel possess the property of retaining air in their meshes, as flannel, the removal of heat is not as rapid as when linen is worn.

_Observation._ In winter, although more heat is generated in the system than in summer, yet we require more clothing, and also those articles that are poor conductors of heat, because caloric is more rapidly extracted in clear, cold weather, than in a warm day.

What does the preceding remark explain? 579. What suggestion respecting the condition of the skin before taking a walk or ride in a cold day? Why? 580. Do the amount and kind of clothing affect animal heat? What is said of well-clothed persons? When does the system generate the most heat?

581. _The health and const.i.tution influence the generation of heat._ When the health is firm, and the const.i.tution vigorous, less clothing is needed, for the change among the particles of matter is more rapid, and more heat is generated, than when the opposite condition obtains.

Persons of a feeble const.i.tution, particularly, if any of the vital organs[18] are diseased, need more clothing and require rooms of a warmer temperature, than individuals who are free from disease and have a vigorous const.i.tution.

[18] The brain, lungs, heart, and digestive organs, are called _vital_ organs.

_Observation._ Persons who are infirm, and whose vital powers are feeble, in general, accustom themselves to an undue amount of clothing and warm rooms. A more judicious practice would be, to exercise more and use a moderate amount of clothing, together with a more nutritious diet.

582. _The surplus heat should be removed equally from all parts of the system._ The rapid evaporation of fluids, as in free perspiration, or from radiation, as in a cold atmosphere, is attended with a removal of heat from the system. This modifies the action of the circulatory vessels. Consequently, if heat is suddenly and rapidly abstracted from one part of the system, the equilibrium of the circulation is destroyed, which will produce disease.

Why do we, then, require more clothing in winter than in summer? 581.

Why do persons of firm health and vigorous const.i.tutions need less clothing than those who are feeble? What is a general practice among infirm persons? What would be more judicious? 582. Why should the surplus heat be removed equally from all parts of the system? What is said respecting currents of air from small apertures?

_Observation._ Currents of air that impinge upon small portions of the body, as from small apertures, or from a window slightly raised, should be avoided. They are more dangerous than to expose the whole person to a brisk wind, because the current of air removes the heat from the part exposed, which disturbs the circulation of blood and causes disease, usually in the form of "colds." For the same reason, it is not judicious to stand in an open door, or the opening of a street.

583. _The system suffers less when the change of temperature is gradual._ The change in the production of heat, as well as in the evaporation of fluids from the system, is gradual when not influenced by foreign causes. This gradual change is known under the name _acclimation_. By this means the body is enabled to endure tropical heat and polar cold. Owing to this gradual adaptation of the system to different temperatures, we can bear a greater degree of heat in the summer between the tropics, than in the winter under the polar circles. On the other hand, we can endure a greater degree of cold in winter and in the arctic region, than in the summer and in equatorial countries.

584. The sensation of heat which would be oppressive in a mild, warm day of January, would only be grateful in July, and a degree of cold which could scarcely be endured in August, would not be uncomfortable in December. The changes of season in our lat.i.tude prevent the disagreeable and perhaps fatal consequence that would follow, if no spring or autumn intervened between the severity of winter's cold and the intensity of summer's heat. During the transition periods, the const.i.tution is gradually changed, and adapted to bear the extremes of temperature without suffering. The amount of heat generated in the nutrient capillary vessels, is likewise diminished or increased as the temperature of the season becomes greater or less.

583. In what manner should change of temperature take place, to be adapted to the body? How is the body enabled to endure tropical heat and polar cold? State some of the effects of the gradual adaptation of the system to different temperatures. 584. What is said relative to a warm day in winter? To a cold day in summer? What is said of the changes of seasons in our lat.i.tude? What effect on the const.i.tution during spring and autumn? What change in the amount of heat generated?

585. But, on the contrary, we cannot suddenly pa.s.s from one extreme of temperature to the other with impunity. Let an inhabitant of Quebec suddenly arrive in Cuba in February, and he would suffer from languor and exhaustion; after becoming acclimated to this tropical climate, let him suddenly return to Quebec in January, and the severity of the weather would be almost insupportable.

_Observations._ 1st. Experience shows that heated rooms, as well as tropical climates, lessen the generation of heat in the body, and likewise the power of resisting cold. It would be idle for the merchant from his warehouse, or the mechanic from his heated shop, to attempt to sit on the box with a coachman, with the same amount of clothing as his companion, who is daily exposed to the inclemency of the weather.

2d. "It is the power of endurance of cold at one period, and the absence of its necessity at another, that enables animals, in their wild and unprotected state, to bear the vicissitudes of the seasons with so little preparation in clothing, and so little real inconvenience."

585. What effect on the system has a sudden transition from a cold to a warm climate? What does experience show? Why do wild animals bear the vicissitudes of the seasons with so little preparation in clothing?

CHAPTER XXIX.

THE VOICE.

586. The beautiful mechanism of the vocal instrument, which produces every variety of sound, from a harsh, unmelodious tone, to a soft, sweet, flute-like sound, has, as yet, been imperfectly imitated by art. It has been compared, by many physiologists, to a wind, reed, and stringed instrument. This inimitable, yet simple instrument, is the _Larynx_.

587. Incidentally, the different parts of the respiratory organs, as well as the larynx, are subservient to speaking and singing. The tongue, nasal pa.s.sages, muscles of the fauces and face, are agents which aid in the intonation of the voice.

ANATOMY OF THE VOCAL ORGANS.

588. The LARYNX is a kind of cartilaginous tube, which, taken as a whole, has the general form of a hollow, reversed cone, with its base upward toward the tongue, in the shape of an expanded triangle. It opens into the pharynx, at its superior extremity, and communicates, by its inferior opening with the trachea. It is formed by the union of five cartilages, namely, the _Thyroid_, the _Cricoid_, the two _A-ryt-enoid_, and the _Ep-i-glottis_. These are bound together by ligaments, and moved by muscles.

586. What is said of the structure of the vocal instrument? With what instrument have physiologists compared it? What is the vocal instrument called? 587. What organs are called into action in speaking beside the larynx? 588-596. _Give the anatomy of the vocal organs._ 588. Describe the larynx. Name the cartilages that form the larynx.

589. The THYROID CARTILAGE is the largest of the five, and forms the prominence in the front of the neck, called _Pomum A-dami_, (Adam's apple.) It is composed of two parts, and is connected with the bone of the tongue above, and with the cricoid cartilage below.