THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS. 23 



Contractility. Nutrition and (with the above-mentioned 

 partial exception) reproduction characterize all living creat- 

 ures; and both faculties are possessed by the simple nucleated 

 cells already referred to as found in our blood. But these 

 cells possess also certain other properties which, although not 

 so absolutely diagnostic, are yet very characteristic of living 

 things. Examined carefully with a -microscope in a fresh- 

 drawn drop of blood, they exhibit changes of form independent 

 of any pressure which might distort them or otherwise mechani- 

 cally alter their shape. These changes may sometimes show 

 themselves as constrictions ultimately leading to the division 

 of the cell ; but more commonly (Fig. 15*) they have no such 

 result, the cell simply altering its form by drawing in its sub- 

 stance at one point and thrusting it out at another. The 

 portion thus protruded may in turn be drawn in and a pro- 

 cess be thrown out elsewhere ; or the rest of the cell may col- 

 lect around it, and a fresh protrusion be then made on the 

 same side ; and by repeating this manoeuvre these cells ma 

 change their place and creep across the field of the micro- 

 scope. Such changes of form from their close resemblance to 

 those exhibited by the microscopic animal known as the 

 Amceba (see Zoology) are called amoeboid, and the faculty in 

 the living cell upon which they depend is known in physiol- 

 ogy as contractility. It must be borne in mind that physiol- 

 ogical contractility in this sense is quite different from the 

 so-called contractility of a stretched india-rubber band, 

 which merely tends tp reassume a form from which it has 

 previously freen forcibly removed. 



Irritability. Another property exhibited by these blood- 

 cells is known as irritability. An Amoeba coming into con- 

 tact with a solid particle calculated to serve it as food will 

 throw around it processes of its substance, and gradually 

 carry the foreign mass into its own body. The amount of 

 energy expended by the animal under these circumstances is 

 altogether disproportionate to the force of the external contact. 

 It is not that the swallowed muss pushes-in mechanically the 

 surface of the Amoeba, or burrows into it, but the mere touch 

 arouses in the animal an activity quite disproportionate to the 

 exciting force, and comparable to that set free by a spark 

 falling into gunpowder or by a slight tap on a piece of gun- 

 cotton. It is this disproportion between the excitant (known 



* P. 48. 



