Possible reasons for the high resistance of muscle to cancer.

Muscle, particularly striated muscle, is highly resistant both to primary and to metastatic cancer. This resistivity is thought to be connected with the lactic acid producing activities of tumors. Lactic acid is an anoxia signal in the body, to which blood vessels tend to respond with a sprouting reaction, new vessels seeking out the source of anoxia and vascularising it. The reaction of the body to incipient cancer is probably two-fold. Fibroblasts treat them as foreign bodies and attempt to encapsulate them, while blood vessels tend to perpetuate them by supplying them with nutrients. The fate of the tumor may be decided by the relative speed of the two reactions. Muscles are lactic acid producers themselves, hence their blood vessels must be conditioned to a greater tolerance of it than in other tissues. This may be the crucial factor in preventing incipient tumors from establishing themselves in muscle.