MECHANISM OF PROTEIN HYDROLYSIS AND IMMUNIZATION 73 



inoculation is found in the spinal cord of a rabbit that has been 

 successively inoculated with the virus of rabies. 



Equally suggestive in their way were the experiments of Was- 

 sermann, who mixed tetanus toxin with the brain substance of 

 a susceptible guinea pig and found that the mixture was no longer 

 toxic for other guinea pigs. This seemed to show a special 

 affinity between the brain substance and the toxin, inasmuch as 

 emulsions of other organs of the guinea pig when brought in 

 contact with the tetanus toxin exerted no such effect. "It would 

 appear from this experiment," says Nuttall, "that a toxin may 

 have a special affinity for special tissue cells, and this appears 

 to explain the neuro-toxic character of the symptoms which are 

 observed in tetanus." 



It would be better, perhaps, to say that certain cells have an 

 elective affinity for the toxin, rather than to make the converse 

 statement; best of all to think of the attractions as mutual. In 

 chemical terminology some cells have side-chain groups of mole- 

 cules for which the molecules of the toxin can readily be sub- 

 stituted ; or with which they may be combined through a new 

 arrangement of the atoms. What determines these affinities we 

 shall not know until we are better informed as to the ultimate 

 nature of chemical valences in general ; but for the present pur- 

 poses it suffices to note the seeming demonstration that such 

 elective affinities exist, and that they are exercised by various 

 tissue cells of the body in connection with an endless variety 

 of nitrogen compounds. 



Another very striking illustration of such elective affinities, 

 in this case involving the muscle cells of the unstriped muscles, 

 is furnished by the amino-acid-like product, known as adrenalin. 



The origin of this product in the supra-renal glands, and its 

 extraordinary effect in constricting arterioles, are familiar to the 

 profession. But it may not be so generally known that adren- 

 alin differs by only two hydrogen atoms in chemical composi- 

 tion from one of the familiar amino-acids named tyrosine. Such, 

 however, is the fact, the formula of tyrosine being CgN^NOg, 

 and that of adrenalin C 9 H 13 NO 3 . That adrenalin exercises its 

 contractile function by directly influencing the muscle fibers of 

 the arterioles is shown by the continuance of its characteristic 

 action when applied locally after severance of the nerves leading 

 to the tissue under treatment. Let it be observed, too, that it is 

 the cells of the unstriped muscles alone that seize on the adren- 

 alin molecule; the cells of other muscles seeming to let it pass 

 unnoticed. 



When we add that adrenalin exists normally in the blood only 

 in the proportion of one part in a million, yet that its presence 



