The osmotic pressure of colloids although it is very small corn pared with that of solutions of electrolytes, has been shown by Starling to play an important part in the formation of urine. Oedema also, according to some authors, is a phenomenon of osmosis.
The chemical behaviour of colloidal particles is very inert compared with the same amount of material in true solution.
This permits of the use in medicine of colloidal preparations of metals and other powerful drugs to secure a slow and prolonged action of continuous minute doses of the active ionized form into which they break down.
The reference already made to the properties of colloids and electrolytes and to some of the parts they play in the animal body, are properties and phenomena which demanded for their elucida tion the use of the microscope, ultra-microscope, polarimeter, electrometer, etc.
Physics of Muscle and Nerve.—The study of the action of muscles and nerves illustrates perhaps in a still more definite manner, the dependence of medical on physical science. This dependence is twofold, in that the investigation and rational ex planation of physiological phenomena has had to await, not only the development of physical theory for the realm of inanimate matter, but also the invention and technical perfection of de tecting and recording instruments of the greatest sensitivity and highest precision.
In order to maintain living muscles and nerves outside the animal for long periods they are bathed in an artificial saline solution, which, as we have seen, must be isotonic with them and have the same hydrogen ion concentration. Now much of the difficulty in investigating vital processes is due to the micros copic size of the living cell, which results in changes which are also small. Largely owing to the development of sensitive gal vanometers and also to the modern thermionic amplifier and cinematograph camera, it is possible to detect extremely small variations of energy if they occur in the form of electrical changes or can be converted into that form. Minute changes of temperature—and therefore the production of small quantities of heat—can be measured by the electrical currents generated in thermocouples or by changes in electrical resistance of metallic conductors.
Methods of Investigation.—In studying the activity of an isolated nerve or muscle preparation, it is usual to employ a brief electric current as stimulus, since it is possible to repeat it with out producing permanent damage and the duration and intensity can be so readily and accurately controlled. By these means, it has been possible to measure with some accuracy the heat pro duced in a muscle by a single stimulus, and also that liberated over long intervals at rest, or in recovery after prolonged stimula tion. The results prove that oxidation is necessary to maintain the normal life and structure of an animal cell in a living but completely resting condition. Further it has been shown that muscular contraction is a surface phenomenon, since the energy developed is proportional to the length of the fibres and not their volume.
Similarly it has been shown that the nervous impulse is ex cited by a stimulus which in all probability consists in an accumu lation of ions in certain parts of the nerve fibre. The arrival of the impulse at any point on the nerve is indicated by the active region developing a negative electrical potential with regard to the neighbouring inactive regions. If then, electrodes are placed in contact with the active and inactive regions and connected through a galvanometer, a current will pass through the circuit towards the active region. It is found that the energy involved in the transmission of the impulse is derived from that stored in the fibre itself, and not from the stimulus: the latter only acts as a trigger.
Radiation.—Not the least service rendered by physics to medicine is the demonstration and exploitation of the fact that, in addition to those radiations—light and radiant heat—which are directly perceived through the senses, there exist others which have a profound influence on the animal body. The spectrum of electromagnetic radiation is now known to comprise, in descend ing order of wave lengths, the Hertzian waves of wireless com munication, infra-red rays, the visible spectrum, ultra-violet rays, X-rays and the gamma rays of radio-active substances. The shorter wave lengths of the visible spectrum and the still shorter ones mentioned above, all possess the power of either stimulating or destroying the animal cell, according as the quan tity of energy absorbed is small or excessive. The action of the rays is selective in that different cells exhibit different degrees of response to a given quality and intensity of radiation.
Some radiations, notably those from certain parts of the ultra violet spectrum, have a specific action on biological material, i.e., the absorption of a given amount of radiant energy of this wave length will provoke a much greater response in a given cell or organism than the absorption of an equal amount of energy of another wave length.
These two effects—the specific and the selective effects of different radiations—are of great importance in radiation therapy.
The mechanism of the action of radiation on the cell has not been traced in all its stages. Ultimately, however, it is attribut able either to the heat produced by the energy absorbed, or to the ionizing (photo-electric) effect on the atoms of the cells in volved. The principal methods of physical dosage measurement in radiation therapy also depend on these two last mentioned properties. Exact dosage measurements have shown that the biological reactions produced by different kinds of radiation obey similar laws in that they all require a minimum duration and have a threshold value of intensity below which no effect is produced.
For other applications of physics to medicine the reader should consult the following articles :—PHYSIOLOGY ; HYGIENE;