8 IMMUNITY IN HEALTH 



mammals and one that only attains its full form late 

 in development. 



The vermiform appendix and the tonsils therefore 

 neither represent a persistent foetal structure as does 

 Meckel's diverticulum, nor do they exhibit the atro- 

 phied condition of the muscle of the external ear. More- 

 over, unlike these relics, they are so often the point of 

 serious disease that they could hardly be expected to 



Fig. 2. 

 Microscopical section of a human vermiform appendix Avith com- 

 plete obliteration of the lumen, showing large thick-walled blood 

 vessels, fat-spaces and fibrous tissue in place of the mucous membrane 

 and lumen. The tunica serosa and the two muscle coats are not 

 specially abnormal in appearance. (From a camera lucida drawing.) 



survive if their presence had no counterbalancing ad- 

 vantages. Treves, in his classic Hunterian Lectures 

 of 1885, stated that he never found congenital absence 

 of the human appendix,^ but in three out of one 



* The Morphological Museum of Columbia University contains two 

 apparently genuine specimens of congenital absence of the human 

 vermiform appendix (see Huntington). But the condition is extremely 

 rare. 



