24 THE HUMAN BODY 



tion: the assimilative or chemically constructive processes are also 

 named anabolic, and the dissimilative katabolic. 



Contractility. Nutrition and (with the above-mentioned partial 

 exception) reproduction characterize all living creatures ; and both 

 faculties are possessed by the simple nucleated cells already re- 

 ferred 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 mechanically alter their shape. These changes may 

 sometimes show themselves as constrictions ultimately leading to 

 the division of the cell; but more commonly (Fig. 99*) they have 

 no such result, the cell simply altering its form by drawing in its 

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

 portion thus protruded may in turn be drawn in and a process be 

 thrown out elsewhere; or the rest of the cell may collect around 

 it, and a fresh protrusion be then made on the same side ; and by re- 

 peating this manoeuver 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 microscopic animal 

 known as the Amoeba (see Zoology) are called amoeboid, and the fac- 

 ulty in the living cell upon which they depend is known in physi- 

 ology as contractility. It must be borne in mind that physiological 

 contractility in this sense is quite different from the so-called con- 

 tractility of a stretched india-rubber band, which merely tends to 

 reassume a form from which it has previously been forcibly removed. 



Irritability. Another property exhibited by these blood-cells is 

 known as irritability. An Amoaba coming into contact 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 mass 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 guncotton. 



* P. 266. 



