Pathology

The reaction by the uninjured parts of the body is the means by which the effect of the injurious agent is diminished and the normal constitution of the injured part restored as far as may be possible. It may be local or general. Many of the symptoms and signs of disease are due to it rather than to the injury which is the beginning of the disease.

Cellular Processes.

In the higher animals, cells which are mechanically injured or have suffered gross structural change seem as a rule to be incapable of recovery : nerve cells may have their long processes cut off and be able to repair the damage, but such facility is quite exceptional. It is more frequent in lower forms of life but here, too, in unicellular animals such as bacteria any substantial injury is usually irrecoverable and is made good not by the restoration of the individual but by a compensatory increase of the other members of the community of which the injured cell formed a part. Similarly in man, when a part is injured, the whole of the body is available and may be used in co operation to bring the part, and with it the whole, back to normal ity. This reaction, whatever its real nature, has the appearance of being purposive in the sense that it is adapted to achieve a definite end.

Pathological changes indeed illustrate exquisitely two great biological truths. The first is that it is characteristic of all live organisms to resist any alterations in their structure and corn position and, if these are varied by circumstances which are be yond control, to proceed actively to correct the deviation and so return to the normal state. The second is that all the parts of which the organism is composed work together and in relationship to one another for the common good. A growing realization of these principles has been one of the most productive changes in recent pathological thought. It carries the corollary that the anatomical changes, which are generally the first objects of study, must be regarded as representing phases of processes rather than definitive states. The mode of thought which has collected into museums specimens of morbid anatomy and classified them as if they were species of animals should be replaced by one which re gards a hypertrophied heart, a cirrhotic liver, or a sclerosed kidney, not as a "typical" example of a state, but as an illustration of one stage in a process by which the body has responded to a stimulus of injury.

Mechanism of Reaction.

The general nature of the reaction being purposive and restorative, it is likely that the particulars of the mechanism by which it is brought about will vary widely, since the constants of live organisms are the ends which are achieved rather than the means by which they are reached. Such is the case as will be seen from some illustrative examples.

The human economy is adjusted to live in air at a pressure of about 76o mm. of mercury containing 21% of oxygen: a com promise of the conflict between having a blood of a tolerable viscosity for the heart to pump round the body and one which will carry an adequate amount of oxygen from the lungs to the tissues has been found at a concentration of about 5,000,000 red corpuscles per cubic millimeter ; and the respiratory centre is adjusted to respond to a pressure of carbon dioxide which cor responds to about 40 mm. of mercury in the alveolar air. If man goes to an altitude of 15,000 ft. above sea-level, where the bar ometric pressure is only some 44o mm. of mercury and the oxygen pressure correspondingly reduced, he is at once in difficulties in respect to the supply of oxygen, especially if the need for it is increased by activity. No clear injury is inflicted on the tissues of the body which can be recognized by anatomical changes, but the shortage of oxygen causes plain symptoms and initiates a re sponse the effect of which is to correct the deficiency wholly or in part. This is done by altering the sensitivity of the respiratory centre so that it responds to a lower pressure of carbon dioxide and so increasing the amount of air breathed, by changing the composition of the blood so that the haemoglobin will take up more than the normal amount of oxygen at a given oxygen pressure and by stimulating the bone marrow to grow and produce more red corpuscles so that the concentration of haemoglobin in the blood and hence its oxygen-carrying power are increased. The connecting links between the deficiency of oxygen and this com plex reaction (i.e., the immediate stimuli to which the reactions are responses) are not known. The result is that man can soon live comfortably under conditions so different from those to which he is accustomed that they would cause serious harm if not compensated.