98 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



A discussion of these different theories, none of which can be regarded 

 as entirely satisfactory, cannot be entered upon here. 



D. CONTRACTILITY. 



Contractility is the property of protoplasm by virtue of which the cell is 

 able to change its form when subjected to certain external influences called 

 irritants, or when excited by certain changes occurring within itself. The 

 change of form does not involve a change of size. The contraction is the 

 result of a change in the position of the more fluid parts of the cell-protoplasm, 

 and the effect is to cause the cell to approach a spherical shape. In the case of 

 an amoeba, for instance, excitation causes a drawing in of the pseudopods, and 

 as the material in them flows back into the cell the body of the cell expands 

 and acquires a globular form. In the simpler forms of contractile protoplasm 

 the movement does not appear to be limited to any special direction, but in the 

 case of the highly differentiated forms, such as muscle, both contraction and 

 relaxation occur on definite lines. 



When a muscle is excited to action, energy is liberated through chemical 

 change of certain constituents of the muscle-substance, and this energy in some 

 unknown way causes a rearrangement of the finest particles of the muscle-sub- 

 stance, and the consequent change of form peculiar to the contracted state. 

 When the irritation ceases and relaxation takes place, there is a sudden return 

 of the muscle-substance to the position of rest, either because of elastic recoil or 

 of some other force at work within the muscle itself. That the recovery of 

 the elongated form peculiar to the resting muscle is not dependent on external 

 influences is evidenced by the fact that a muscle floating on mercury, and 

 subjected to no extending force, will on the cessation of irritation assume its 

 resting form. The relaxation no less than the contraction must be regarded 

 as an active process, but on account of their flexibility muscle-fibres are incap- 

 able of exerting an expansion force, therefore cannot by relaxing do external 

 work. 



Both the histological structure and physiological action of the striated mus- 

 cles which move the bones show them to be the most highly differentiated, the 

 most perfect form of contractile tissue. It is by means of these structures that 

 the higher animals perform all those voluntary movements by which they change 

 their position with reference to external objects, acquire nourishment, protect 

 themselves, and influence their surroundings. An exact knowledge of the 

 method of action of these mechanisms and the influences which affect them is 

 therefore of the greatest importance to us. 



1. Simple Muscle -Contractions Studied by the Graphic Method. 

 When a muscle makes a single contraction, in response to an electric shock or 

 other irritant, the change of form is too rapid to be followed by the eye. To 

 acquire an adequate idea of the character of the movement it is necessary that 

 we should obtain a continuous record of the alterations in shape which it un- 

 dergoes. This can be done by connecting the muscle with a mechanism which 

 enables it automatically to record its movements. 



