IRRITABILITY. 21 



sion be then made on the same side; and by repeating this 

 manoeuvre these cells may change their place and creep 

 across the field of the microscope. Such changes of form 

 from their close resemblance to those exhibited by the micro- 

 scopic animal known as the Amoeba (see Zoology) are called 

 amoeboid, and the faculty in the living cell upon which they 

 depend is known in physiology as contractility. It must 

 be borne in mind that physiological contractility in this 

 sense is quite different from the so-called contractility of 

 a stretched Indian- rubber band, which merely tends to re- 

 assume a form from which it has previously been forcibly 

 removed. 



Irritability. Another property exhibited by these blood- 

 cells is known as irritability. An Amoeba coming into 

 contact with a solid particle calculated to serve it as food 

 will throw around it processes of its substance, and grad- 

 ually 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 mass 

 pushes-in mechanically the surface of the Amoeba, or bur- 

 rows 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 gunpow- 

 der or by a slight tap on a piece of gun-cotton. It is this 

 disproportion between the excitant (knotfnin Physiology as 

 a stimulus) and the result, which is the essential character- 

 istic of irritability when the term is used in a physiological 

 connection. The granular cells of the blood can take 

 foreign matters into themselves in exactly the same man- 

 ner as an Amoeba docs; and in this and in other ways, as 

 by contracting into rigid spheres under the influence of 

 electrical shocks, they show that they also are endowed 

 with irritability. 



Conductivity. Further, when an Amoeba or one of these 

 ^lood-cells comes into contact with a foreign body and 

 proceeds to draw it into its own substance, the activity ex- 

 cited is not merely displayed by the parts actually touched. 

 Distant parts of the cell also co-operate, so that the influ- 



