Page:Life in Motion.djvu/172

152 means a diminished power of work. Up to a certain point, the substances produced in a working muscle are got rid of as quickly as they are formed and new materials are supplied for the repair of the muscle. There is thus a balance between the two processes. But if a muscle is made to work for a long period, or if it is excited to very frequent contractions, the waste products gather or accumulate in the muscle, and sufficient time is not allowed for the supply of reparative material. Muscle, like most other tissues, is richly supplied by a special set of vessels, of very minute size, called the lymphatics, which are for the purpose of draining away the excess of nutrient matter that has oozed out of the vessels, along with the waste matters that have been formed. If the waste stuffs are produced too quickly—such stuffs as carbonic acid, acid phosphate of potash, and the stuffs of a nitrogenous nature, such as kreatin and other bodies found in extracts of meat—the muscle becomes fatigued; it consumes less oxygen and produces a smaller amount of waste products.

It is very instructive to watch how a muscle behaves as it becomes tired. I have