Observation has shown, first, that lactic acid is eliminated chiefly by the skin, and second, that it is formed during the action and retro grade metamorphosis of the tissues of the motor apparatus. The effects which we should expect to result from an excess of lactic acid in the blood, are, therefore, increased action of the skin, and, failing that, functional disturbance of the motor apparatus. And that is exactly what Dr. Foster observed.
Lactic acid is so readily oxidized and eliminated, that it is only in exceptional cases that ingestion can exceed elimination. Hence it is only in exceptional cases that its internal administration can give rise to disturbance. One of these exceptional cases is that recorded by Dr. Foster. The patient was suffering from diabetes and plithisis. —both of them ailments accompanied by imperfect oxidation. " In diabetes," says Dr. Foster, "we have a state of disordered nutrition very unfavorable to the conversion by oxidation of new compounds ; and iu Wright's case this was aggravated by the serious pulmonary complications. Associated with this was a dry branny state of the skin highly unfavorable to the elimination of the lactic acid by one of the common channels." Lactic acid given under such circumstances—given, that is, to a man in whose system it cannot be oxidized, and by whose skin it cannot be eliminated—must be retained, and cause disturbance of the nutrition of the textures of whose metamorphosis it is a product. An excess of lactic acid in the blood checks the retrograde metamorpho sis of these textures, and so disturbs their whole nutrition, just as retained excreta produce a corresponding action on the brain, and as an atmosphere of carbonic acid interferes with the elimination of that gas from the system. The symptoms resulting from the re tention of lactic acid in. the blood are, therefore, likely to be those of functional disturbance of the tissues of the motor apparatus. Func tional disturbance declares itself in different ways in different organs. In the brain it pauses nervousness, irritability, headache, giddiness, delirium, convulsions, and coma. In the heart it gives rise to more or less disturbance of the rhythm and force of its action. In the digestive organs it declares itself by evidences of imperfect and de ranged digestion. In fibrous tissue the evidence of its existence is pain, which may be very severe, and is well exemplified by what is felt when a ligament is unduly stretched, or when, as in acute rheu matism, it is the seat of inflammation.
A disturbing agency like lactic acid, which acts on both the mus cular and fibrous textures, will declare itself chiefly by symptoms referable to the latter. It will do so, because these textures give more ready and decided evidence of functional disturbance; and be cause any weakening of the muscles to which it might give rise would be lost in the muscular debility and wasting characteristic of the diabetes for which the acid was given.
Pain localized in the muscular and fibrous tissue of the motor ap paratus is, therefore, the chief symptom which we should expect to find follow the retention in the blood of an excess of lactic acid. And so in reality it is found to be. That is the true interpretation of Dr. Foster's facts. His evidence shows that lactic acid causes increased action of the skin; and failing this, disturbed nutrition of, and con sequent pain in, the muscles, and in the fibrous textures of the joints ; but it goes no further.
But it may be said, if you grant this—if you admit that excess of lactic acid in the blood may give rise to articular pains—do you not thereby admit the truth of the lactic-acid theory of rheumatism? By no means; the two propositions are not only not identical, but are widely different. That excess of lactic acid in the blood is the cause of some of the phenomena of acute rheumatism, is a position which I not only admit, but am prepared to maintain. But that is a very different thing from saying that it causes all its phenomena, which is what the lactic-acid theory asserts. That theory is, that lactic acid is the rheumatic poison, the special materies morbi which initiates and sets agoing all the phenomena of the disease. One of the most con stant of these phenomena is the presence in the blood of an excess of lactic acid. But lactic acid cannot be the cause of its own increased formation—of its own excess. The position is an absurd one; but its very absurdity serves to demonstrate the weakness of the lactic acid theory, and to bring before us the real nature of the question which we have to consider, and of the difficulties which we have to face.