Pathology

Response.

Of the circumstances which facilitate response, age and experience are the most important : their influences will often be antagonistic. The inherent capacity of a tissue to grow and dis charge any of its functions steadily diminishes as the organism of which it forms a part grows older. But its available ability to do anything is influenced by whether and how often it has done it before, individually and racially. Experience in such exercises as reproducing blood which has been lost by haemorrhage or making the substances which neutralize bacterial poisons gives the same advantage as it does in the more complicated reactions of behaviour. In general, the most effective preparation for suc cessful resistance to a large injury is the previous reception of small injuries of the same kind.

Reactions to stimuli may fail in whole or in part because the stimulus is too large and kills the whole organism or because the tissues are old or already diseased. In some instances it is the nature of the tissue which mostly determines the imperfection of the result. Blocking of one of the smaller blood vessels of the heart kills the muscle which the vessel normally supplies. The dead muscle autolyses and is removed, and its place is taken by fibrous tissue instead of by cardiac muscle, which is inherently incapable of the necessary multiplication of cells. The repair is for the time effective but it is unsatisfactory, for fibrous tissue is less elastic than muscle and is liable to tear. The nerve cells of the central nervous system never grow to repair an injury : if some of them are destroyed they cannot be replaced, and the best that can be achieved is a certain amount of functional restoration by the rearrangement and adaptation of the parts which remain. In other cases, the nature of the 'injury and the mode of its infliction deter mine the result. The common sclerosis of the kidney begins by the destruction of a proportion of the glomerulotubal units of which the organ is composed by some poison circulating in the blood. The dead tissue is cleared away and, since kidney cells cannot adequately regenerate, the gap is healed by the growth of the connective tissue. Later, which is characteristic of the disease, further destruction takes place and so on from time to time, each loss of secreting tissue being followed by fibrous tissue repair.

The other units, as might be expected, hypertrophy, and up to a point function is adequately maintained, partly by the local re action and partly by a general rise of arterial blood pressure backed by a hypertrophied heart which tends to compensate the diminution of vasomotor mobility and control resulting from the anatomical changes. But if successive injuries continue, the limits of the possible hypertrophy of the remaining healthy tissue are exceeded and its growth is impeded by the firm fibrous tissue which comes to constitute a large part of the organ. Hence, in the end, function fails, as it finally must if repeated injuries are inflicted on tissues which cannot be vicariously replaced and are capable only of compensatory hypertrophy (i.e., most specialized tissues) and not of indefinite regeneration (e.g., connective tissue, blood corpuscles).

These examples are sufficient to illustrate the kind of topics which form the subject-matter of pathological study and the way in which they can at the moment be fruitfully approached. The reaction to injury (using the word in a wide sense even including the new growths or tumours) is a process which is initiated by some definite stimulus arising within or outside the body and generally promoted and extended by secondary stimuli which are produced by the body in the early stages of the response. The injury and the response may be local or general ; general injuries may provoke reactions, and local injuries may elicit responses in which many organs take part. The organism in this, as in other respects, has to be regarded as a coherent and essentially in divisible whole, and one must recognize the limitations of the point of view, fruitful enough in the past, from which a live animal appears as an assemblage of relatively independent cells or organs. The character of the response is such that it tends to correct in one way or another the anatomical defect or the functional derangement which is caused by the injury. Whatever the mechanism by which the result is attained, the reaction is purposive in appearance if not in intention, and its purpose is survival.

See BACTERIA AND DISEASE ; PARASITIC DISEASES ; CANCER RESEARCH; TUMOUR; and articles on special diseases or diseases of special organs.