The Comparative Auatomy of the Palatine Tonsil

1 The tuberculum tonsillare is always present in the embryo, but its subsequent history depends on the amount of lymphoid tissue which is formed in it. 2 Epithelial ingrowths are usually formed in the palatine tonsil at some period of growth. 3 Special investigation was made of the condition in the dog and cat. It was found that (a) epithelial ingrowth takes place in the palatine tonsil of the dog during the fœtal period and first few months of post‐natal life, probably continuing to some extent throughout life; (b) epithelial ingrowth occurs to a small extent in the palatine tonsil of the Domestic Cat during the fœtal period and first few days of post‐natal life only. 4 The extent of epithelial ingrowth in the palatine tonsil is correlated with (a) activity of the tonsil, (b) the location of the lymphoid tissue. Epithelial ingrowths are less evident in simple pocket‐shaped tonsils, and are also inversely proportional to the size of the tuberculum and to the amount of lymphoid tissue which it contains. 5 Tonsils may be grouped into five classes:— a. Simple pocket‐shaped tonsils with epithelial ingrowths only in the early stages, e. g., rabbit. b. Pocket‐shaped tonsils with numerous ingrowths and crypts, e. g., Felidæ. c. Tonsils with a flattened sinus and large projecting tuberculum tonsillare in which is located most of the lymphoid tissue, e. g., Canidæ. d. Tonsils with flattened sinus and no tuberculum tonsillare in the adult lymphoid tissue developing below the floor of the sinus, e. g., Artiodactyia. e. Tonsils with a flattened sinus and small tuberculum tonsillare, the lymphoid tissue being formed on the floor of the sinus and usually projecting from it as a solid mass, e. g., Man and anthropoid apes. 6 It is suggested that certain limiting folds surrounding the palatine tonsil serve to direct the secretion downwards in the fauces. 7 No clear correlation can be demonstrated between tonsil form and feeding‐habits, but the following generalisations may be made:— a. In carnivorous mammals (both Eutherian and Marsupial) the palatine tonsil usually constitutes a solid projecting mass. b. In herbivorous mammals the palatine tonsils are either of a simple pocket‐shape, as in the Rodents, or they are extensive and of the “embedded” type, being developed below the surface‐level of the pharynx. c. In insectivorous mammals the palatine tonsil is relatively small and simple. It is hence suggested that the size and importance of the tonsil in the adult may have some connection with its action in freeing the pharyngeal mucosa from food particles.