By this term is meant a simple diminution in the nutritive activity of the structures, and a consequent diminution in size without further change. Strictly speaking, we should distinguish from this a small-ness due to defective growth, to which the terms Hypoplasia and Aplasia are applied. But atrophy is frequently used so as to include both conditions.
There are certain normal processes of decay which occur in the body. At certain periods, for instance, the milk teeth drop out, and this is effected by an atrophy of the fang so that the crown is shed. At a still earlier period the thymus gland atrophies. Then again throughout life there is a continual shedding of the hair. If a cast-off hair from the eyelash be examined under the microscope, it will be seen that its bulb is atrophied, and this is the cause of its being shed. In some persons the hair of the scalp is largely shed at a comparatively early age, without being properly reproduced, there being here an atrophy of the hair-sheath and papilla. Then there is the normal atrophy of the tissues generally, which occurs in old age. The atrophy of old people is in many cases due to some organic disease whose symptoms are not manifest; but we are all familiar with the healthy old person with shrivelled hands and face and plicated skin.
In all these cases there is a kind of intention in the tissues, so to speak, according to which they live a certain period and then decay. As Paget has pointed out, such atrophies may almost be regarded as active processes. The fall of the leaf is due to an active absorption or atrophy of the fibres uniting it to the stem; if the leaf dies before its time, or is killed, it remains hanging, but in the natural course it drops when its time is come. So with our tissues and the whole organism, there is a limit to their activity. The period varies in different persons, and in this respect hereditary influences have an important bearing. Just as these largely determine the period of growth of the body and its rapidity, so do they influence the duration of activity of the tissues. This is plainly seen in the case of the hair; baldness runs in families, just as longevity does.
Atrophy in many cases depends on some interference with the supply or alteration in the quality of the nutritious material supplied to the tissues. It is also related frequently to diminution in the function of the parts, while in some cases it depends more directly on interference with the nervous arrangements.
General emaciation indicates that the tissues generally have been affected in such a way as that their nutrition is diminished. This will occur as the result of an alteration in the blood. The blood, being the vehicle for the conveyance of nutriment to the tissues, may be impoverished because of a direct interference with the food-supply, as in starvation, in stricture of the oesophagus, excessive vomiting, diarrhoea, etc.; or there may be an excessive consumption of the nutritious material, in cases of excessive discharges, as in phthisis pulmonalis, or ulcerating cancers; or there may be, as in fevers, an increased consumption of the nitrogenous elements of the tissues.
In general emaciation the various constituents of the tissues do not atrophy in an equal degree. According to experiments by Chossat, in which animals were deprived of food, the fat and blood diminished most, next to them came the muscles and the abdominal glands, while the bones and central nervous system diminished least.
Senile atrophy is closely related to the physiological atrophies already mentioned. The atrophy of some tissues in old age renders them more liable to pathological processes. Thus the bones, being diminished in size, and having proportionately less animal matrix, are more liable to fracture; and the lungs, having lost their supporting tissue, are more liable to emphysema (senile emphysema). The brain also suffers atrophy in old people, and the kidneys frequently do so.
Atrophy from disuse manifests itself chiefly in the muscles and glands. When a joint is disused from being rendered rigid by disease or from paralysis, the muscles undergo atrophy, and even the bones diminish if the condition be prolonged.
Atrophy from pressure is exemplified in the atrophy which occurs as a consequence of the advance of tumours and aneurysms, or external pressure from stays, etc. In the last mentioned case the liver frequently suffers considerable atrophy. Atrophy is also not uncommon in organs which are the seat of interstitial inflammation, the new-formed connective tissue by its direct pressure on the proper tissue, or by obstructing the blood-vessels, causing atrophy.
This is a somewhat wide subject and will be more fully considered in the special part of this work. In ordinary motor paralysis there is atrophy from disuse. But there are atrophies of a more active kind following lesions of nerves and of the spinal cord which are referred to interference with the trophic nerves or centres. These include atrophy of the nerve-fibres and of the muscles. According to Charcot a muscle or nerve atrophies when cut off from its trophic centre, and this may be effected by interruption of the conductivity of nerve-fibres, or by destruction of the centre. The atrophy of the muscles in lead-palsy belongs to this class.
Hemiatrophy of the face belongs to the class of neurotic atrophies. It affects the soft parts of the one half of the face and of the tongue, while the bones are not affected unless the disease has occurred in early life, and even then the bones are unequally affected. The lesion is-probably due to interference with the nerves, perhaps in their passage through the cranium or at their ganglia. A hemiatrophy of the body sometimes occurs from cerebral lesions in the foetus or young child. This may be in the form of a crossed hemiatrophy, the face and extremities being affected on opposite sides.
In the various forms of atrophy, with the exception of general emaciation, the proper functionating tissue is that which chiefly suffers diminution. Thus in muscles it is the contractile substance, in glands the secreting cells, in nerves the nerve-fibres, which are specially affected. The atrophy of the proper tissue is often accompanied by new-formation in the accessory structures. Thus, atrophy of muscle is often associated with increase of the interstitial connective tissue. This connective tissue frequently becomes the seat of fatty infiltration, so that adipose tissue largely replaces the muscle, bringing about a pseudo-hypertrophy. (See under Hypertrophy and Fatty Infiltration.) A similar process frequently occurs in and around disused glands.
Paget, Surgical Path., 3rd ed., p. 69; Recklinghausen (Neurotic atrophy), Allg. Path., p. 326; Charcot, Senile diseases (Syd. Soc. trans.), 1877, Dis. of nerv. syst. (Syd. Soc. trans.), 1881; Chossat, Rech. exp. sur l'inanition, 1843.