Apart from their duties of devouring the inside layers of bones and clearing away dead tissue, they are supposed by some to assist in the absorption of food by creeping between the cells lining the alimentary canal, and, after throwing out arms to engulf particles of food, returning with their spoils into the body. Perhaps, however, the most interesting, or at any rate most romantic, of their many and important functions are what may be called their emergency duties. Frequently people, especially those who live in smoky towns, draw into their lungs particles of dust and soot, which if left adhering to the walls of the air cavities would cause dangerous irritation. As if by magic a leucocyte will discover the presence of such a nuisance, and, crawling between the cells forming the wall of the lung, in which, by the way, it is outside the body proper, will engulf it and carry it away with him. This exploit, however, pales beside the warfare which goes on in the body between leucocytes and invading bacteria. A bacterium thrives in the blood or lymph, since it finds itself in a warm alkaline fluid containing complex organic substances, by breaking down which it can easily obtain energy. Unfortunately, the products of such a process are frequently virulent poisons, the effect of which upon neighbouring cells produces the distressing symptoms which we associate with disease. No sooner, however, has the bacterium begun to generate poisons, than leucocytes, influenced by chemical attraction (Essay I.), swarm upon it. First come leucocytes of a small kind, full of zymogen granules, which crowd round the bacterium till they have covered it. After a time they creep away, leaving it dead. They are now in an exhausted condition, and no longer contain granules, having doubtless discharged them as a destructive ferment upon their enemy. Then a leucocyte of another kind moves to the attack, or, rather, to clear up the remains, for he is a large, non-granular, active fellow, and eats up the dead bacterium by the simple process of engulfing him whole. (See Diagram 31.)