80 



NKTI.KIXAKV PAT I lOLOC.Y. 



almost universally accepted the proposition and have at their 

 command the means by which they can immunize animals 

 against the ravages of some of the fatal infective diseases to 

 which thtv are susceptible. The increased confidence of the 

 people is in turn enabling scientists to investigate new phases 

 of the subject. Althougli immunization has been known and 

 made use of more or less for centuries as vaccination against 



Marsaranus .Vnnulatus, f.niali 



P^iS- 56. — IMargarapiis .Annulatus, male. 



I'i^^ 57. — Marsarapus Annulatus 



fi-nialr. l-iyhi,!^ fc,E;h'^. 



-3Iar.t;arapus Annulatus, T.arva. 



smallpox 1)\' the Chinese l)efore the Christian era, yet the essen- 

 tial ph}-siologic, chemic or pathologic basis for immunity is 

 still unknown. 



Immunity may be natural (inherited) or artificial (accjuired). 



Natural immunity is an inherited propertv possessed bv or- 

 ganisms (animalsV Tlie horse has a natural immunitv to hog 

 cholera, the ox to glanders and the hog to tick fever. The 

 concise and exact cause of natural immunitv is unknown. It 

 is probablv the result of cellular activity in the immune animal, 

 an activitv the nature of \vhich is not understood. Some inves- 

 tigators, IMetchnikoff in particular, attribute natural immunity 

 to pliagocytosis ("cellular hypothesis), others maintain that insus- 



