178 GENERAL PROPERTIES OF LIVING TISSUES 



Muscle fibres consist of contractile substance im- 

 bedded in sarcoplasm. The relation between 

 the contractile substance differs in the same 

 muscle in different species and individuals, and 

 differs further in the muscles of the same indi- 

 vidual. In striated muscles of vertebrates, those 

 rich in sarcoplasm have a turbid, opaque .appear- 

 ance, while those poor in sarcoplasm are translu- 

 cent. Important differences in contractility, 

 irritability, etc., depend on this difference of 

 structure. Muscles which contain many " clear " 

 fibres (poor in sarcoplasm) are more irritable 

 than those containing many of the fibres rich in 

 sarcoplasm. In the flexors of the frog the " clear " 

 fibres are relatively more numerous than in the 

 extensors. 



Specific Irritability of Nerve Greater than that 

 of Muscle. Arrange an inductorium for single 

 induction currents. Make as rapidly as possible 

 two nerve-muscle preparations, A and B. Bring 

 a wire from the secondary coil to each end of 

 muscle A. Let the nerve of B rest on muscle A. 

 No stimulation can now reach B except through 

 that part of the nerve of B which rests on muscle 

 A. Place the secondary some distance from the 

 primary coil. Stimulate muscle A with make 

 induction shocks, the strength of which is gradu- 

 ally increased by approximating the coils. 



