For 1 grm. HO at 30° to 40° requires 610 to 600 calories). example: Temperature of air, 20°; thermometer on forearm, 30°; hygrometer covering 20 sq. cm. of skin absorbs 0.5 milligramme H2O per minute. (By previous graduation it has been determined that each degree above surrounding temperature, indicated by the thermometer strapped to a bag of water cooling at a known rate, signifies a heat-emission of 0.0125 cal. per sq. cm. per min.) The surface of the forearm (not including the hand) is 540 sq. cm. ; its heat-emission is then : By radiation and conduction . + by evaporation 0.0125 × 10 × 540=67·5 75.8 cal. per min. or, again, e.g. after exercise, the thermometer is at 34°, and the hygrometer absorption is 2.5 milligrammes per minute, the emission is then: By radiation and conduction . 0.0125 × 14 × 540=94·5 + by evaporation . 0·61 × 2·5 x 540 = 41.2 135.7 cal. per min. Simple clinical thermometry, although from a physiological point of view yielding very complex results, is, nevertheless, of FIG. 114.-NORMAL TEMPERATURE CHART OF THE HUMAN BODY DURING great practical value if carefully performed with reliable instruments, and not committed to the charge of careless assistants. Taken with other indications, the temperature is valuable evidence of the presence and degree of fever, and of its probable course. But, to be of use, the observation should be taken twice or four times in the twenty-four hours, and entered for reference upon a temperature chart. The graphic curve of several days is of far more value than a single isolated observation. In view of startling statements occasionally made, it is not superfluous to mention that clinical thermometers should possess an index in working order, that the index should be shaken to below the normal point before use, that any suspected instrument should be re-verified, and, finally, that a malingerer may know how to send a thermometer to fever-heat by rubbing it against the blankets. Other memoranda are-the normal daily fluctuations of temperature, the effect of digestion and of exercise, the variations with sex and age. Normally, temperature rises during the day and falls during the night, being thus at its highest at the end of the day, at its lowest at the end of the night. Normally, again, the temperature is raised after a meal and after exertion -more so, indeed, on a weak than on a healthy subject. Finally, the temperature is less constant in women than in men, and least constant in children. A high temperature is, as a rule, more significant in a man than in a woman, and in a child it may possibly signify nothing more than a recent fit of crying. The part selected for thermometry may also influence the result; in the axilla the normal reading is 37°, in the mouth 37-2°, in the rectum 37.6°, and the last-named is the most trustworthy. After an animal has been killed, its body-temperature does not at once begin to fall, like that of an inert mass which has been heated to the same degree. The internal temperature may even rise after death, because with arrest of the circulation and respiration loss of heat has been arrested, or at any rate greatly diminished, while the organs and tissues have still continued to produce heat. This post-mortem rise is not solely attributable to rigor mortis, for it makes its appearance sooner; it is a manifestation of the residual metabolism of the tissues and organs of the body, and an expression of the fact that their death does not coincide with that of the organism-the individuals outlast for a time the dissolution of the community. No doubt muscle takes a chief share in such effect, and in so far as it does so its rigor must contribute to delayed cooling; we find, moreover, that in cases where rigidity appears early and strongly (e.g. tetanus) the post-mortem rise and the delayed fall of temperature are most pronounced. But even in such cases rigor is not the sole factor; it is only one factor in the general tissuevivacity of which it is a sign. On the other hand, if death supervenes as the termination of chronic disease, if at the death of the body the tissues are moribund, there is no post-mortem rise of temperature, and the cooling of the body is but little delayed, in comparison with that of an inert mass. Deaths from fever afford instructive illustration of the varying thermic effects of residual metabolism; the tissues of a fever patient dying at a high temperature are active, and by continuing active retard the cooling of the body after death; the tissues of a fever patient dying at a low temperature are already moribund, and have but little retarding effect upon the further cooling of the body. 289 Excitability-The reflex arc-Reflex action. 291 Cortical and medullary centres-Brain, bulb, and cord-Voluntary (automatic) and reflex acts Master' centres and foreman' centres. 298 Why and How? PROTOPLASM is excitable. When any part of a lump of protoplasm is excited, the lump moves. When many lumps of protoplasm are gathered into a homogeneous mass, excitations and movements may be transmitted from lump to lump in all directions. With higher organisation of the mass, differences of function and of structure begin to make their appearance. Excitability, while still pervading the whole organism, becomes localised with greater intensity in some parts than in others, along some lines more than along others (sense-organs, nerves, and nerve-centres); in other parts contractility becomes the salient character (muscles). To illustrate this progressive elaboration of a nervous system we may select-(1) an amoeba, (2) a jelly-fish, (3) a frog, (4) a man. An amoeba is a simple lump of protoplasm, excitable and contractile in all parts of its substance, and not more so or less so in one part than in another. The lower surface of the umbrella of a jelly-fish is covered with a layer of contractile protoplasm, which is not far removed from a homogeneous state; an excitation applied to any part causes a contraction, which is transmitted in all directions through the mass; it is a sheet with neuro-muscular properties, a network of rudimentary muscle and nerve. U A frog exhibits a far higher degree of differentiation. Instead of a neuro-muscular cell we have a comparatively complex chain of parts, different in structure and in function, muscle-fibre exhibiting contractility as its salient characteristic, nerve-fibre and nerve-cell possessing conductivity without contractility. In such an animal we recognise the component parts of a nervous system as central, peripheral, and intermediate, and, as we shall see, we may further analyse these components into cerebral and spinal centres, into afferent and efferent conductors, and into many kinds of end organs in connection with these conductors. The progressive elaboration of a nervous system with subdivision of function has its highest expression in the human brain. There is in the human brain, and in less degree as we go down the scale, in the mammalian brain, a localisation of different functions in different parts. We shall find that this localisation is best known in the cases of language, of voluntary motion, and of sight, and that as regards voluntary motion in particular, it is possible to trace a correspondence between particular movements and the functional activity of particular areæ of the cerebral cortex. The literal meaning of the word excitation is call from without.' The surroundings of an organism ex-cite' its specially excitable parts, and the organism moves to or from its surroundings, or registers an impression which will modify its future movements. The specially excitable parts are on the external surface, exposed to excitation; they have no direct communication with the specially contractile parts-the muscles, but an indirect communication by strands of specially excitable protoplasm-nerve-fibres, which conduct excitations to central organs, where they are received, while other nerve-fibres establish the communication from the nerve-centres to the muscles, conducting from the former to the latter the impulses that give rise to movements. The central nervous cell is usually considered as a double cell, one part of which is at the central end of an afferent fibre, the other half at the central end of an efferent fibre. The first is called a sensory cell, the second is called a motor cell; and we must imagine that a bond of union (commissural fibre or fibrils) establishes communication from sensory' to motor cell (but see p. 543). The component parts just enumerated form a reflex arc, and may be classified as central, peripheral, and intermediate. The central part is a nerve-centre or centres, the peripheral parts are the sensificatory organs and the muscles, the intermediate parts are the nerves-afferent and efferent-and the fundamental nervous act is a reflex act. In relation to the conduct of the animal body, the sense-organs are the intelligence department, nerve-centres are the headquarters, muscles are the executive, nerves are the channels of communication. In the ascending scale of organisation there is increasing diversity of parts, increasing subdivision of properties and of functions, with further diversities between the parts of parts. The simple nervous system below-figured is not that of a mammalian animal nor of man. The nerve-centre' of mammalia and of man is a collection of nerve-centres occupying the cerebrospinal axis, with more or less diverse special offices under their control-communicating each with the other upon occasions, yet separately active upon other occasions-having functions that are localised at certain parts, yet not strictly confined to these parts-playing upon and influencing each other in all directions, yet in some directions rather than in others, and maintaining some kind of precedence and rank, so that while all may influence all, yet some are usually guided and controlled by othersvariously organised through past excitations, yet still variously organisable by excitations to come. To-day the state and disposition of organs and of the organism are the product of the past, immediate and remote, individual and ancestral. Tomorrow and in the distant future they will become what they may be made to become by training, by education, and by new conditions of life. Classification of nerve-centres.-The cerebro-spinal centres |