PHYSIOLOGY OF MOVEMENT. 



719 



The irritability of muscle is subject to great variation, and a single 

 irritant is, under different circumstances, capable of producing different 

 results. In an excised muscle the irritability rapidly disappears, although 

 it persists much longer in the muscles of cold-blooded animals than in 

 warm-blooded. In the latter case the irritability of warm-blooded 

 muscles may be somewhat prolonged if the animal be artificially cooled 

 before death. 



Temperature is of great influence on the irritability of muscle, both 

 an increase or decrease above the normal temperature of the muscle being 

 followed by a decrease in irritability. Again, through repeated contrac- 

 tion the muscle loses its power of being thrown 

 into contraction ; it is then said to be in a con- 

 dition of fatigue, due either to the insufficient 

 supply of nutritive substances or to the accumu- 

 lation of the products of decomposition, which, 

 as has been experimentally demonstrated, pro- 

 duce a hurtful action upon the muscle-fibres. 

 In all probability both of these factors are 

 concerned in fatigue. 



When a contracting muscle is examined 

 under the microscope marked changes in 

 structure may be made out. If a living mus- 

 cular fibre of an insect, for example, which is 

 especialty fitted for such study, be examined 

 under the microscope while contracting, a wave 

 of contraction, as already mentioned, may be 

 seen traveling along the surface of the fibre, 

 while at the same time the transverse striations 

 approach each other. In the contracted portion 

 each disk has become shorter and broader, while 

 the band which in a relaxed muscle is light, 

 in a contracted muscle becomes dark, and the 

 band which in a relaxed muscle was dark in the 

 contracted muscle becomes light (Fig. 285). 



In the process of contraction chemical changes occur ; these have 

 been already to a certain extent indicated. The muscle in which reaction 

 was alkaline, in contraction becomes acid through the development of 

 sarcolactic acid, which may even be excreted by the kidneys. During its 

 contraction the muscle absorbs more oxygen from the blood than during 

 its stage of rest, and, as a consequence, venous blood from a resting 

 muscle contains 8.5 per cent., that from a contracting muscle 12.8 per 

 cent, less oxygen than the arterial blood ; while the venous blood 

 from a resting muscle contains, as an average, 6.7 per cent., that from a 



FIG. 285. MUSCULAR FIBRE 

 UNDERGOING CONTRAC- 

 TION, AFTER ENGL.E- 

 MANN. (Foster.) 



The muscle is that of Telephorus 

 melanurus treated with osmie acid. 

 The fibre at c is at rest, at a the con- 

 traction begins, at b it has reached its 

 maximum. The right-hand side of 

 the figure shows the same fibre as 

 seen in polarized light. 



