212 VETERINARY PHYSIOLOGY 



tone may be overcome by force, but that the limb then 

 remains fixed in the position in which it is placed. This has 

 been termed plastic tone. 



It has been found that the rate of chemical change in 

 muscle in decerebration tonus is low when compared with 

 the rate in ordinary contraction (p. 266). There seems to 

 be some difference between the processes. It has been 

 suggested that the sarcostyles are the essential elements in 

 contraction, and that the sarcoplasm may be responsible for 

 tonus, but evidence is wanting. 



Some have maintained that the tone of the muscles is 

 due to the presence of visceral fibres in the motor nerves 

 (fig. 105). Such fibres have been demonstrated in many 

 muscles, e.g. in the extrinsic muscles of the eye after the 

 third nerve has been cut. Certainly tone depends upon the 

 integrity of the reflex arc (p. 82). 



(4)) Heat Production. — Muscle, like all other living proto- 

 plasm, is in a state of continued chemical change, constantly 

 undergoing oxidation and reconstruction. As a result of 

 this chemical change, heat is evolved. But the heat 

 evolved by muscle at rest is trivial when compared with 

 that evolved during contraction, and heat production will 

 later have to be considered fully (p. 266). 



(5) Electrical Conditions. — (1) Uninjured resting muscle, 

 when at rest, is iso-electric, but if one part is injured, it acts 

 to the rest like the zinc plate in a galvanic battery — becomes 

 electro-positive. Hence, if a wire passes from the injured to 

 the uninjured part round a galvanometer, the needle is 

 deflected, indicating a current passing along the wire from 

 the uninjured to the injured part ; just as, when the zinc 

 and copper plates in a galvanic cell are connected, a current 

 is said to flow through the wire from copper to zinc 

 (fig, 106). This is the Current of Injury. In investigating 

 the electrical condition of muscle, non-polarisable electrodes 

 must be used. 



(2) "When a muscle contracts certain electrical changes 

 occur. These may best be studied in the ventricle of the 

 heart of a frog, which is a muscle which can be exposed 

 without injury. 



