274 THE HUMAN BODY 



adrenal bodies. It exhibits very characteristic properties and is 

 thought to be the hormone secreted by them. The physiological 

 action of adrenalin appears to be very specific, and to be that of 

 stimulating the post-ganglionic fibers of the sympathetic system 

 proper at their terminations in smooth muscles or in glands. 

 Curiously those post-ganglionic fibers that arise in other than true 

 sympathetic ganglia, as the ones supplying the ciliary muscle of 

 the eye, appear to be unaffected by adrenalin. 



The widest distribution of smooth muscles receiving innerva- 

 tion from the sympathetic system proper is in the circular coats 

 of the small arteries. Accordingly, the most conspicuous effect of 

 the administration of adrenalin is upon the circulation, through 

 changes in the caliber of these vessels. This effect will be con- 

 sidered in detail in connection with the circulation (Chap. XXII). 



Infection. Bacteriology has taught us that we are continu- 

 ally 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 are always finding their way into the lymph spaces 

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

 genic 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 infection are not due to 

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

 or toxins, which the bacteria produce as incidents in their vital 

 processes. Some sorts of bacteria 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 them- 

 selves, and it is only when they die and decompose that the poison 

 is liberated. 



Resistance to Infection. It is obvious that the Body does not 

 become infected every time pathogenic organisms gain entry into 

 it. If it did infection would be our hourly fate. The tissues of 

 the Body form, however, excellent culture media; bacteria that 



