THE STRUCTURE OF VOLUNTARY MUSCLE 



u 



sheath ends suddenly, while the axis cylinder ramifies in a maTof ' 



differentiated protoplasm, containing nuclei, 



and lying in contact with the contractile 



substance of the muscle immediately under 



the sarcolemma (Fig. 40). This mass of 



protoplasm is known as the ' sole plate.' It 



is not marked in all animals. Thus in the 



frog the axis cylinder ends in a series of 



branches at right angles to one another, dis- 



tributed over a considerable length of the 



muscle fibre. The sole plate in this case 



seems to be limited to scattered nuclei lying 



in close contact with the terminal branches 



of the nerve fibre. So far as we can tell at 



present, the ultimate ramifications of the 



axis-cylinder end freely and do not enter 



into organic connection with the contractile 



substance itself. 



Most of our knowledge on the subject of muscle 



lias been derived from the study of the gastroc- 



nemius and sartorius muscles of the frog. The 



position of these muscles is shown in the accompany- 



ing 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 only be employed when the contraction of 



the muscle as a whole is the object of investigation. The effective cross-area of tin- 

 fibres is much greater than the actual cross-section of the muscle, so that, while tin- 

 actual shortening of the gastrocnemius is but small, its strength of contraction is 

 considerable. 



The sartorius 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 tin- (lntrli. 

 arising from the symphysis pubis by a thin flat tendon, and is inserted by a narrou 

 tendon into the inner side of the head of the tibia. On account of the regularity wit 1 

 which its fibres are disposed, this muscle is of especial value in experiments on t he !<>< -al 

 conditions of a muscle fibre accompanying its activity. When a greater mass of ap- 

 proximately parallel fibres is necessary, recourse may be had to a preparation consis 

 of the gracilis and semi-membranosus muscles together. This latter mu.sele lu-s <loi>ally 

 to the gracilis muscle which is shown in the illustration. 



FIG. 49. Meter end-organ of a 

 lizard, gold preparation. (Ki ' n M:. , 

 n, nerve fibre dividing as it ap- 

 proaches the end-organ ; r, ramifi- 

 cation of axis cylinder upon b, gran- 

 ular bed or sale of the end-organ ; 

 m, clear substance surrounding the 

 ramifications ef the axis cylinder. 



