CHAP. xiv. POPULAR DISCOVERIES IN PHYSIOLOGY. 145 



tance to the organism seem to be now made clear. 

 They are much smaller and less numerous than the red 

 blood-globules; they move about quite independently; 

 and they behave in a manner which shows that they are 

 closely allied to, if not identical with, the amrebse found 

 abundantly in stagnant water, and which form such 

 interesting microscopic objects. These minute animal 

 organisms, which inhabit not only our blood-vessels "b.ut 

 all the tissues of the body, have an important function 

 to perform on which our very lives depend. This func- 

 tion is, to devour and destroy the bacteria or germs of 

 disease which may gain an entrance to our blood or tis- 

 sues, and which, when their increase is unchecked, pro- 

 duce various disorders and even death. Under the 

 higher powers of the microscope the leucocytes, as they 

 are termed, can be observed continually moving about, 

 and on coming in contact with any of these bacteria or 

 their germs, or other hurtful substances, they send out 

 pseudopodia from their protoplasm which envelops the 

 germ and soon causes it to disappear; but they also ap- 

 pear sometimes to produce a secretion which is injurious 

 to the bacteria, and so destroys them, and these may per- 

 haps be distinct organisms. 



It seems probable, and, in fact, almost certain, that so 

 long as we live in tolerably healthy conditions, these 

 leucocytes (or phagocytes as they are sometimes called 

 from their function of devouring injurious germs) are 

 able to deal with all disease-germs which can gain access 

 to our system; but, when we live in impure air, or drink 

 impure water, or feed upon unwholesome food, our sys- 

 tem becomes enfeebled, and our guardian leucocytes are 

 unable to destroy the disease-germs that gain access to 



