408 



THE PROPERTIES OF STRIPED MUSCLE. 



parallel-fibred and others. In some parallel-fibred muscles the fibres 

 are as long as the whole muscle. Each fibre is provided with one or 

 (in the frog) more nerve-endings, and these are disposed either near 

 the middle, or on either side of it ; none occur at the ends. Consequently, 

 when the muscle is excited through the nerve, the excitatory influence 

 reaches the middle of the muscle first. The tendons extend so short 

 a distance into the muscle, that muscular fibres underlie the surface 

 everywhere. Of such a muscle the sartorius is a type. If this muscle 

 is prepared with due care, its surface is found to be nearly equvpotential, 

 i.e., no appreciable current is indicated by a galvanometer of which the 

 terminal electrodes are applied to its surface, whatever the situation 

 of the contacts. In some parallel-fibred muscles, as in the adductor 

 internus or gracilis and the semi-membranosus, the fibres are interrupted 

 by tendinous " inscriptions " (see Kg. 189), the presence of which, as will 

 be seen subsequently, modifies the electromotive properties of the 



organ. Of muscles in which the fibres are 

 not parallel, the gastrocnemius is the most 

 important example. In this muscle more 

 than half the surface is tendinous, and the 

 fibres spring from an incomplete median 

 dorso- ventral tendinous plate, of which the 

 border is seen on the ventral surface of 

 the muscle, dividing that surface into an 

 outer and an inner half. From this plate 

 as origin, the fibres sweep round, to be 

 inserted dorsally in the deep surface of a 

 tendinous expansion of the tendo Achillis 

 (Fig. 222). Each fibre in this case receives 

 a single nerve-ending. In the gastro- 

 cnemius and other muscles of similar struc- 

 ture, considerable differences of potential 

 The letter/is on the dorsal present themselves, especially between the 

 surface, close to the edge of muscu i ar an d tendinous parts of the sur- 



tue tendinous expansion ; „ -r, „ • i • .1 •, -n 1 



s, on the ventral surface, fac e. Before considering these, it will be 



marks the edge of the dorso- advantageous to describe the general method 



ventral tendinous plate. In f examining a muscle galvanometrically. 



the third figure this plate is ° ° 



seen in section. Method of observation. — In English 



laboratories a " Thomson " galvanometer of 

 high resistance (8000 to 20,000 ohms) is used. The galvanometer is 

 provided with a shunt, by means of which Jg-, t ^q, or T oVo respectively, of 

 any current may be led through the coils. The circuit may be arranged 

 according to some such plan as is shown in Fig. 223. From the principal 

 terminals of the shunt, n and s, the wires lead to the north and south terminals 

 of the galvanometer. When the plug between n and s is withdrawn, the 

 galvanometer is in the circuit, but with a plug in the position marked £ in the 

 figure, only one-tenth of the current in the circuit flows through the galvano- 

 meter. When the keys K, K l , and A' 4 are plugged, the circuit is completed 

 through the plugs. When the plug of A is withdrawn, it is completed through 

 the preparation pr d, the wire from d leading to the north terminal of the 

 galvanometer, the wire from pr to the south terminal. When the key A' 1 

 is unplugged, so much of the wire B C as intervenes between C and the 

 sliding contact is included in the circuit. The wire stretched between 

 the two blocks O and B is a rheochord through which, when the key A 3 is 

 plugged, the current of a battery flows. B C is a metre long, and is 



Fig. 222 



Diagram showing the 

 arrangement of the fibres in 



the 



gastrocnemius muscle. 





