PHYSICS IN MEDICINE. The science of physics con cerns itself with energy in its various manifestations and the properties of matter, and in consequence plays a part in medicine whose value and importance are ever increasing. It furnishes means of investigating the structure of the animal body and the functions of its members; it assists in the detection of abnormality and disease, and in the preparation, control and administration of therapeutic remedies ; finally, it enables man to acquire a measure of control of his environment by scientific heating, light ing, ventilation and drainage, and the development of means of transport for himself and his necessities.
In particular, such processes of the living body as muscular contraction, the circulation of the blood, the respiratory intake and output of the lungs, the nervous impulse and its journeyings and the maintenance of the energy balance by food intake are in a measure explicable in terms of physics and physical chemistry.
To consider some of these in detail, first the question of the energy value of food may be cited. According to the principle of conservation of energy—if this is valid for transformations of energy occurring in the living animal —the same amount of energy should be evolved from the utilisa tion of food inside the body as outside, provided the resulting chemical end-products are the same in the two cases and have the same physical state. This has been shown to be the case by the direct measurement over a sufficiently long period, of the heat given out by resting man or animal when placed in a suitable calorimeter, and by also separately measuring the heat evolved in the oxidation of a quantity of food equal to that which was consumed in this period. When due allowance is made for sub stances which are excreted unchanged or only partially oxidized, and for the work of evaporation, it is found that the agreement is beyond doubt. This is of considerable importance, for if it were not the case, then it would be quite uncertain whether or not man received energy from unknown sources and physics would be of little use in medicine. As it is, the fact that the
oxidation of food is the only source of bodily energy of man, makes possible the prescription of diets adequate to a variety of conditions, whether of health or disease.
Not only does the principle of conservation of energy apply to the body as a whole, it is also applicable to the units which go to form it, i.e. to all those energy changes and oxidations necessary for life which occur in the cell. Now the living cell consists of a watery solution containing colloids, crystalloids and substances in suspension, enclosed in a cell wall or membrane. Therefore the study of the physical properties of solutions, colloids and membranes in the laboratory should, and does, afford much in formation concerning its behaviour. Below some of these proper ties are examined with this object.
It is well known that solu tions which conduct electricity (electrolytes) do so in virtue of the electrically charged "ions," positive and negative, derived from the dissociation of the molecules of the dissolved substances. Pure water is only slightly ionized, yielding hydrogen ions carry ing a positive electrical charge and an equal number of hydroxyl ions with a negative charge, the product of the concentration of hydrogen ions and hydroxyl ions being constant for a given tern perature. When a substance is dissolved in water it, in general, produces a change in its chemical neutrality, owing to the dis turbance in the balance existing between the concentration of hydrogen and hydroxyl ions. Thus an acid dissolved in water yields hydrogen ions as a direct result of solution, and leads to an increase in hydrogen ion concentration and a decrease in the concentration of hydroxyl ions. Similarly the solution of a base produces hydroxyl ions and leads to a decrease in hydrogen ion concentration and an increase in the concentration of hydroxyl ions.