t>r» IMMUNITY IN HEALTH 



the weak places. Such a disposition would, indeed, 

 interfere with the absorption of food. The frontier is 

 defended, not by a wall, but by a chain of forts which 

 draw the attack. 



There are two apparent exceptions to the rule that 

 the subepithelial lymphoid tissue occurs wherever bac- 

 teria are most abundant. The fore parts of the nasal 

 fossae strain off most of the inspired bacteria, yet there 

 is no tlonsil in the front of the nose. In health, how- 

 ever, the bacteria do not multiply in the nasal mucus. 

 Moreover lymphoid tissue being so prone to enlarge 

 would frequently block the air way, as indeed adenoid 

 tissue often does even in the much wider nasopharynx. 

 The point is referred to in a subsequent Chapter (XVI.). 



Similarly, the mouth, which always contains patho- 

 genic organisms and is extremely liable to septic in- 

 fections (for example, decayed teeth) is devoid of lym- 

 phoid tissue. But it must be remembered that the 

 buccal cavity is lined by thick stratified epithelium and 

 not by a delicate columnar layer. And, of course, 

 the faucial tonsils are immediately adjacent. 



It may be noted before closing this section that the 

 thymus appears originally as subepithelial lymphatic 

 tissue about the third internal cleft recesses and is 

 thus serially homologous with the faucial tonsils. It 

 soon, however, migrates, losing its connection with the 

 pharynx, though epithelial remnants in it persist as 

 " Hassal's concentric corpuscles." 



