306 THE HUMAN BODY 



were grouped into specific masses, recognizable as independent 

 organs. 



Since the hormones whose functions are at all understood are 

 discussed in connection with the bodily processes with which they 

 are associated no further account of them need be given here. 



Infection. Bacteriology has taught us that we are continually 

 surrounded by myriads of micro-organisms of various kinds. 

 They are on the skin and mucous membranes; they are breathed 

 in with the air and swallowed in the food and water; colonies of 

 them flourish in the intestinal tracts. Not all of them are disease 

 producing (pathogenic), but these are always present along with 

 the harmless varieties. 



Not only are these organisms always present, but small num- 

 bers of them frequently find their way into the lymph spaces of the 

 Body, whence they get into the blood. The entry of pathogenic 

 organisms into the Body does not constitute infection. It is only 

 when they gain a foothold and begin to multiply that the infection 

 is established and the disease under way. 



It is recognized that the ill effects of an infection are not due to 

 the presence of the organisms merely, but to poisonous substances, 

 or toxins, which they produce as incidents in their vital processes. 

 Some sorts give off this poison to the blood, themselves remaining 

 out of the blood-stream; the diphtheria organism is of this sort. 

 Others retain the toxin within themselves, and it is only when they 

 die and decompose that the poison is liberated. 



Resistance to Infection. In order for organisms to attack the 

 Body they have first to get within it. So long as the skin and the 

 lining membranes of the respiratory and digestive tracts are intact 

 the entry of organisms is difficult, if not impossible. A prime fea- 

 ture in the resistance to infection, therefore, is the preservation of 

 the membranes intact. The great danger from an ordinary cold ; 

 which in itself is usually a mild infection, is in the damage to the 

 mucous membranes which accompanies it, and which may afford 

 channels of entry 7 to organisms which otherwise would not be able 

 to gain admission. In ^uninjured membranes, then, we have the 

 " first line of defense" against infection. 



Even though organisms do succeed in penetrating the mem- 

 branes infection does not always or even usually follow. If it did 

 infection would be our fate much more frequently than it is. The 



