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A MANUAL OF VETERINARY PHYSIOLOGY 



piece of isolated nerve, which does not, as does a muscle, change 

 its shape or exhibit other obvious changes when stimulated. 



The electrical change in muscle can be observed without the 

 assistance of any apparatus, by means of an experiment as old 

 as electricity itself. In the hands of Galvani, it was described as 

 electricity without metals. The nerve of a nerve-muscle pre- 

 paration (M, Fig. 123) is brought into contact with another muscle, 

 M', in such a way as to touch at two points— viz., the cut end 

 and the surface of the muscle. The current of injury in the 

 latter travels along the nerve, and produces a contraction in M. 

 With the same preparation it can also be shown that if M' be 

 stimulated at S and made to contract, the negative variation 

 which accompanies the contraction of M' will produce a con- 

 traction of M. This is known as a 

 ' secondary contraction,' while the 

 above preparation is spoken of as 

 the rheoscofiic frog. 



The electrical phenomena in 

 muscle are not an isolated example 

 of electric currents in the body. 

 Closely similar phenomena are de- 

 monstrable in nerves, which we 

 shall shortly study, and electrical 

 changes accompanying their func- 

 tional activity occur in secreting 

 glands, in the eye, and to the highest 

 degree in the electrical organs of 

 certain fish. These electrical organs 

 are modified muscle or skin glands. 

 The Changes in Active and Rest- 

 ing Muscles. — The physiological 

 changes occurring in muscles are remarkably active. The pro- 

 cesses which result in muscular contractions use up at every 

 moment the combustible material of their structyre, and the 

 products arising from their metabolism have to be got rid of at 

 once and repair brought about. Changes are also constantly 

 occurring even during the period of muscle rest. Muscle activity 

 is characterised by muscle waste, muscle rest is characterised by 

 a preponderance of the process of repair ; we must therefore 

 learn the nature of the waste and repair occurring in muscles. 



The oxygen carried to resting muscles by the blood is absorbed 

 in considerable quantities, and a volume of carbon dioxide, in 

 slightly less quantity than corresponds to the oxygen absorbed, 

 is returned to the venous blood. Whether a muscle is at rest or 

 active, it is always absorbing and storing up oxygen, and giving 

 off carbon dioxide. The absolute amount of these varies ; during 



Fig. 123. — Secondary Con- 

 traction. 



