MUSCULAR CONTRACTION 15 



of hydrogen ions, such as Hill's hypothesis demands, produces 

 no measurable bio- electric effect. 



(/) The Phenomenon of Tonus. — ^An aspect of muscular 

 contraction which is still very obscure is exemplified in a rather 

 striking manner by certain contractile mechanisms of inverte- 

 brates. In the living animal a skeletal muscle is not normally 

 relaxed completely, as in the isolated nerve-muscle preparation. 

 The partial contraction of skeletal muscle in situ is maintained 

 by the C.N.S. The opposite is the case with smooth muscle 

 which when isolated from the body remains in a state of tonus, 

 and in the intact animal is subject almost universally to control 

 by nerves whose action is inhibitory. Smooth muscle is 

 capable of maintaining this state of tonus for very long periods 

 without any appreciable sign of fatigue. Thus Parnas (19 10) 

 found that the adductor of the mollusc Dioxinia can keep its 

 shell closed for twenty or thirty days at a stretch against a 

 tension per sq. cm. of muscle attachment of about two and a 

 half million dynes — a tension exerted in virtue of the elastic 

 cushion which causes the shell valves to fly apart when the 

 muscle is relaxed. This can be brought about, in the freshwater 

 mussel Anodon at least, by stimulating the inhibitory nerves 

 from the pallial ganglion ; section of the nerve supply of the 

 adductors does not lead to relaxation. In Pecten a rather 

 curious phenomenon results from the co-operation of two 

 separate constituents of the adductor muscle, one composed 

 of striated fibres and the other of smooth muscle fibres. When 

 scallops (Pecten) are taken out of the water they usually give 

 one or two flaps of the shell- valves and then close tightly ; if 

 a solid object is interposed between the valves they close on it 

 Hke a vice. But if the foreign body is then made to slide out 

 of position, the valves remain set fast at the same degree of 

 closure. Uexkull (19 12) has shown that on cutting away the 

 smooth muscle, the remainder can be excited to contraction 

 by nervous stimulation ; but the contraction of the striated 

 fibres of the adductor persists only so long as stimulation lasts. 

 The motor portion (striated fibres) of the adductor serves to 

 bring the valves together rapidly, while the more slowly 

 reacting *' catch muscle " (smooth fibres) keeps the valves closed 



