STRIATED MUSCLE. 89 



traction. When the animal is at rest the style is at the 

 maximum of elongation, and the body as far as possible 

 from the points of adhesion and of refuge. In this state the 

 central filament of the style, the contractile fibril, is com- 

 pletely extended ; it is, however, never straight, but always 

 twisted in a long spiral line, like a ribbon wound round 

 its longitudinal axis; which resembles exactly the spiral 

 spring of a watch, fixed and firmly drawn out at the extremi- 

 ties. 



" At the instant that any mechanical, electrical, or thermi- 

 cal excitant touches the animal, this elongated spiral line, 

 returning suddenly upon itself, is transformed almost instan- 

 taneously into a spiral spring of perfect uniformity, the coils 

 of which are very near each other, measuring not much more 

 than a fifth of the length of the style when in repose, while 

 their transverse diameter is proportionately increased. This 

 state lasts generally only for a short time ; the coils of the 

 spring withdraw from each other, the spring lengthens out 

 slowly, and the animal returns to its former position. 



" The shortening or lengthening of the contractile organ 

 is here evidently owing to the junction or separation of the 

 coils of a spiral spring. But which of these two states is that 

 which brings the elasticity into play ? In which do we find 

 the muscular spring returned to its natural form, the state of 

 repose ? In the first place, observation has established this 

 important fact, that the spiral filament is never found at its 

 extreme elongation unless the animal is alive and uninjured. 

 If the animal be killed, or the style detached, the coils of the 

 spring roll up into a tendril, and remain in this state : the 

 case is the same if the animal be killed by any poisonous 

 agent, or by raising the temperature to -f- 40 or 45 (C). It 

 frequently happens, during the lifetime of the animal, that 

 the contractile fibril is severed, and thus the continuity 

 between it and the body, or the trophical centre of the whole 

 animal, broken : in this case, although the sheath is perfect, 

 the body, living and swimming by means of the vibratile 

 cilia, drags at its inferior part the dead contractile fibril, 

 rolled up like a tendril, having for ever lost the power of 

 further elongation. The lengthening of the spiral fibril, 

 the organ of muscular movement in the vorticellus, belongs 

 thus to the living state, that is, to the continuance of nutri- 

 'tion and exchange of matter. As soon as nutrition has been 

 stopped by the death of the animal, or by the separation of 

 the fibril from the trophical centre, the contractile element 



