=Irritability= is a property commonly attributed to protoplasm, but it is a little doubtful whether there be not again some danger of an illogical use of terms. An amœba, one of the unicellular organisms found in ponds, has the power of moving. If a piece of a water-plant—the stalk of duck-weed is a suitable object—be examined with the microscope, these little animals are usually to be found upon its surface. They feed upon algæ more minute than themselves. When they come in contact with something suitable for food, their body-substance flows around it. The food is coagulated. So much of it as is digestible is digested; the remainder is extruded. Constantly parts of the body-substance are protruded, other parts retracted, in the search for food. Such movement is a response to stimulus. Stimuli received at one part of the body-substance are transmitted to another. The body-substance is irritable. It acknowledges stimuli; it conducts them. But if the amœbæ are watched until, owing to lack of oxygen or other cause, they die, their irritability comes to an end. It is a phenomenon of life. Again the physiologist is in a dilemma. Either protoplasm is not protoplasm when death has supervened, or protoplasm is not irritable as such. It is somewhat paradoxical to ascribe to the physical basis of life a property which depends upon its being alive. Yet the influence on protoplasm of anæsthetics makes it difficult to understand how it can be either physically or chemically a substance which loses its form or changes its constitution whenever it ceases to display the usual evidences of its existence. Chloroform and similar agents suspend irritability. Yet irritability returns as their influence passes off. They appear to hold it in check without—at any rate visibly—changing the nature of the irritable substance. All parts of the minute body-substance of an amœba are equally irritable. In higher animals irritability is concentrated in the nervous system. The form of irritability to which consciousness is adjunct is restricted to the cortex of the great brain.