184 PHYSIOLOGY 



Most of our knowledge on the subject of muscle has been derived from the study 

 of the gastrocnemius and sartorius muscles of the frog. The position of these muscles 

 is shown in the accompanyirtg diagram (Fig. 41). The gastrocnemius which, with 

 the attached sciatic nerve, is most frequently employed as a nerve-muscle preparation, 

 forms a thick belly immediately under the skin at the back of the leg, and arises by 

 two tendons from the lower end of the femur and the outer side of the knee-joint. The 

 two tendons converge towards the centre of the muscle, uniting about its middle, and 

 from them a number of short muscular fibres arise, passing backwards and dorsally to 

 be inserted into a flat aponeurosis covering the lower half of the muscle, which ends 

 in the tendo Achillis. On account of this irregular arrangement of the muscular fibres, 

 the gastrocnemius can be employed onlywhen the contraction of the muscle as a whole 

 is the object of investigation. The effective cross-area of the fibres is much greater 

 than the actual cross-section of the muscle, so that, while the actual shortening of the 

 gastrocnemius is but small, its strength of contraction is considerable. 



The sar'torius muscle consists of a thin band of muscle fibres running parallel from 

 one end of the muscle to the other. It lies on the ventral surface of the thigh, arising 

 from the symphysis publis by a thin flat tendon, and is inserted by a narrow tendon 

 into the inner side of the head of the tibia. On account of the regularity with which 

 its fibres are disposed, this muscle is of especial value in experiments on the local con- 

 ditions of a muscle fibre accompanying its activity. When a greater mass of approxi- 

 mately parallel fibres is necessary, recourse may be had to a preparation consisting of 

 the gracilis and semi-membranosus muscles together. This latter muscle lies dorsally 

 to the gracilis muscle which is shown in the illustration. 



Other muscles in the frog used for particular purposes are the mylohyoid and the 

 dorsocutaneous muscles. The mylohyoid muscle of the frog, which lies on the ventral 

 surface of the tongue, has the advantage that its fibres lie in close contact with a lymph-, 

 space occupying the centre of the tongue. If any drug be injected into this lymph - 

 space it acts with extreme rapidity on the muscle fibres, so that the tongue-prepanit ion 

 of the frog is a useful one for the study of the action of different substances on muscle- 

 fibres. 



