PREDISPOSITION, IMMUNITY, AND DISEASE, 381 



ance to the foe a word which, does not actually explain the mat- 

 ter, but furnishes a convenient term. Germs of disease are to be 

 found everywhere, but only predisposition permits its develop- 

 ment. Immunity, on the contrary, is the condition of the system 

 which prevents an outbreak. The fundamental cause of this con- 

 dition is as little known as the cause of predisposition ; only in 

 a few cases have we been successful in tracing it back to certain 

 chemical and physiological processes occurring in the body. 



The action of carbon monoxide on different animals affords a 

 suitable instance of what is called immunity, and illustrates the 

 kind of circumstances on which it may sometimes depend. Car- 

 bon monoxide is an air-like compound, which is contained to a 

 large amount in the illuminating gas produced by the decompo- 

 sition of steam by red-hot coals, and to the presence of which the 

 poisonous qualities of this gas are chiefly due. A mixture of one 

 part of carbon monoxide and ninety-nine parts of common air, 

 when breathed, will in a short time kill any of the warm-blooded 

 vertebrates. Cold-blooded vertebrates, such as frogs, can for a 

 considerable length of time stand the exposure to such an atmos- 

 phere ; arthropoda or insects are not in the least affected by it 

 they possess immunity from it. Searching for the cause of these 

 differences of effect, we find it to be the tendency of haemoglobin, 

 the albuminous matter constituting the red corpuscles of the 

 blood, to combine with carbon monoxide. In the process of res- 

 piration in warm-blooded animals haemoglobin takes up oxygen, 

 which thereafter, as a necessary agent in the exchange of matter, 

 is delivered to the different organs of the body. Carbon mon- 

 oxide prevents the absorption of oxygen, being absorbed in its 

 place ; but, unfit as it is to replace oxygen in its vital functions, 

 it causes serious derangements, which end in suffocation. In 

 cold-blooded vertebrates respiration is of more subordinate im- 

 portance ; although, as well as in warm-blooded animals, it con- 

 sists in absorption of oxygen by haemoglobin, the need for oxy- 

 gen is much lower ; a frog can live for a considerable time with- 

 out the accession of air. Hence the effect of carbon monoxide is 

 a much slower one. The blood of insects contains no haemoglo- 

 bin ; carbon monoxide is not absorbed by it, and is not a poison 

 to them, provided that a sufficient amount of oxygen is always 

 present. Carbon monoxide, consequently, acts as a strong poison 

 upon warm-blooded animals ; its effect is weaker in cold-blooded 

 vertebrates ; and insects are proof against its effects. 



In a few instances only has the cause of immunity become as 

 well disclosed as in the one mentioned. Neither differences of 

 organization in animals nor in the constitution of the poisonous 

 substance generally afford any clew for interpreting an exceptional 

 want of effect. Unaccountable is the immunity of rabbits against 



