The muscular system is rarely the seat of morbid growths, except when it is involved in those which have originated in other tissues.

A. Teleangiectasis

It occurs under the form of more or less extensive convolutions of dilated vessels, by which the muscular substance becomes displaced and atrophied, though at the same time the belly of the muscle retains its natural outline.

B. Formation Of Fat

What is called conversion of muscle into fat, or steatosis musculorum, is a change into a substance resembling adipocere or spermaceti, or into actual fat. The muscles most exposed to it are the voluntary muscles, especially those of the lower limbs: slighter degrees of the same change affect also the muscular substance of the heart, and the fleshy coat of the gall-bladder. There are two forms in which it occurs.

a. In the first, adipose tissue or fat-cysts are formed between the ultimate muscular fibres; in the voluntary muscles the transverse striae1 disappear, and their red color is exchanged for a paler hue, a pale yellowish-red, a dirty yellow, or a dun color: after a time the fibres themselves become disintegrated and disappear, and the muscular tissue gives place altogether to a mass of fat. Small globules of free fat are found as well as fat-cysts; and sometimes there is also a yellow granular pigment.

Many grades of this degeneration have been observed, which run imperceptibly into one another. They are distinguished by the external condition of the diseased muscle, which varies in respect to the amount of discoloration, the degree in which its texture is lost, and the preservation or alteration of its external form.

1 It is not yet made out whether the disappearance of the transverse striae is a primary or a secondary part of the change; but it is probably the former; for they disappear in paralyzed muscles, under the same conditions as those under which steatosis takes place, though steatosis be not present.

The earliest change is one of color. The muscle has a pale reddish appearance; and is found, on close examination, to be not uniformly discolored, but stained irregularly of a yellowish or fawn color, as well as marked with longitudinal pale reddish streaks, which follow the course of the fibres.

As the stains of fat increase in size and coalesce, the muscle acquires an almost uniform fawn color: but its fibrous arrangement still remains distinct.

With the advance of the disease it becomes altogether of the color of fat; sometimes being yellow, sometimes remarkably white, and resembling accordingly either tallow or spermaceti. No trace of its fibrous structure remains except some of its tendon, or the cellular sheaths of its fasciculi.

Up to this stage of the disease the outline of the muscle has been preserved; but in the last stage the mass of fat, into which the muscle is changed, mixes with the adipose structures around it, with the subcutaneous adipose tissue, or with other masses of fat which have been developed by the same process in adjoining structures. We may then find in a limb nothing of its muscles, but remnants of tendons, and aponeuroses with their prolongations inwards.

A muscle thus diseased is usually very thin; and an extremity in which all, or a great part of, the muscles have undergone such a change, is slender as if emaciated, and cylindrical from losing the contour of its muscles: it has also a uniform feeling of toughness.

These are the stages of the disease in question, from that at which it is first distinguishable by the anatomist, to that in which the muscle has completely disappeared, at least so far as opportunities of observing it have occurred to myself and others. In its first and slightest degree it is closely allied to that state of emaciation and pallor in which the animal muscles are found when there is a great and general deposit of fat in the body, such as occurs chiefly in the female sex, and is associated with changes in other organs, which will be mentioned afterwards. It is allied, also, to the state of atrophy of the organic muscular coat of the bowel, and even of the bladder, etc, which is found when large quantities of fat are accumulated in the folds of the peritoneum and cavity of the pelvis.

The process consists, therefore, in the production of a quantity of fat, which compresses the muscular fibres; and the name of fatty metamorphosis of muscle can be applied to it only in so far as the place of the muscle is actually occupied by fat.

Whether the disease occur in one muscle or in many, it usually commences at every part of them at once. Sometimes, however, it appears first at several isolated spots on the surface or in the interior; or it terminates at particular points, as is especially the case in the heart.

It is met with in the substance of the heart, and in the fleshy coat of the gall-bladder, but is most common in the muscles of the lower extremities.

Among the causes which lead to it, we are able to enumerate advanced age, a sedentary luxurious kind of life, the misuse of alcohol, and complete inaction of the muscles, - such inaction as is produced by anchylosis, or by spasmodic contraction of some other muscle.

The fatty degeneration which is brought on by a sedentary and luxurious mode of living, and by spirit-drinking, is, for the most part, accompanied by a great development of fat throughout the system; the liver usually contains a quantity of tallowy substance, the heart is loaded with fat, and its muscular tissue is more or less metamorphosed into the same substance: moreover, in old people more particularly, the medulla of the bones connected with the altered muscles is in excess, and the bones are in a state of osteoporosis, or excentric atrophy, and are easily broken.

The fat by which the muscular tissue is supplanted varies in its character. In some instances it resembles ordinary healthy fat; sometimes, especially in persons advanced in years, it is of a dark yellow color, loose and diffluent; and sometimes, in its consistence and whiteness, it is remarkably like mutton suet. It assumes this last character, particularly when the change results from anchylosis.

There is another form of degeneration which has not been hitherto observed, and to which the muscular structure of the heart is liable, particularly when hypertrophied: it is met with, also, but not so frequently, in the muscular coats of other organs when they are hypertrophied, but is very seldom seen in the muscles of animal life. It is characterized by the development of minute particles of free fat between the primitive muscular fibres. At the same time the striated sheath of the fibre disappears, and the muscle changes to a dirty yellow or fawn color, and becomes friable. I once met with the disease in the muscles of the calf, in which it had given rise to considerable pain: this fact coincides with the experience of other observers.