Now putting these various facts together—finding that many of the fibrous and serous tissues of the body are not subject to rheuma tism, but that the disease is limited almost entirely to such fibrous and serous tissues as are habitually engaged in facilitating, regulating, and restraining active movement—we cannot fail to see that the seat of rheumatism is not fibrous tissue in general but such fibrous tissue as is habitually engaged in controlling and regulating movement; and not serous textures in general but such serous textures as are habitually engaged in facilitating free and active movement. In other words, rheumatism is essentially a disease of the motor appara tus, and the chief seats of the morbid process are the fibrous and serous structures of that apparatus.
There are fifteen common seats of rheumatic inflammation—two hips, two knees, two ankles, two shoulders, two elbows, two wrists, two hands, and one heart, and the one functional characteristic com mon to the fibrous textures of all is that they are habitually engaged in controlling and regulating movement. - They are the only fibrous textures which possess this function.
But though the fibrous structures and synovial membranes are the tissues on which the rheumatic poison produces the most marked effects, they are not the only ones which give evidence of disturbance. The evidence is most pronounced in them because the symptoms which constitute it—pain and swelling—are so prominent and obvi ous. So much do such symptoms force themselves upon the notice of both patient and physician, and so dominant are they during the whole course of the disease, that attention is naturally concentrated on them. But a careful consideration of all the phenomena which go to constitute an attack of acute rheumatism indicates that the muscles suffer at the same time as the joints. Their participation
in the morbid process is indicated by the muscular pains and aching which usher in the attack, by the tenderness to touch of the muscles, and by the rapidity with which their tissue wastes away. Further very distinct evidence of disturbance of the muscles is found in what, next to fever and joint pains, is the most essential and characteristic feature of the disease—excessive formation of lactic acid. This phe nomenon calls for explanation as urgently as the joint inflammation, for it forms as characteristic and as constant a feature of acute rheu matism; acid sweats are almost as essential to the diagnosis of acute rheumatism as joint pains.
Lactic acid is a normal product of the metabolism of muscle. In its quiescent state muscle gives a neutral or feebly alkaline reaction, when actively contracting the reaction is acid. This acid reaction is due to the presence of lactic acid formed during the state of activity.
Lactic acid being formed during muscle metabolism, excessive forma tion of that acid in the system indicates increased metabolism of muscle. As this excess of acid always occurs in acute rheumatism and never in any other disease, it follows that an increase in muscle metabolism is one of the characteristic features of the rheumatic process—one of the results of the action of the rheumatic poison. The consideration of the question of an excess of lactic acid in the system really resolves itself into the consideration of the question as to why muscle metabolism is increased. To this point we shall return by and by. Attention is called to it now with the object of showing that the rheumatic process is not limited to the white fibrous structures of the joints, but affects all their fibrous structures, the muscles as well as the tendons and ligaments.