VESTIGIAL STRUCTURES. 93 



animals, it may be useful to draw attention to one con- 

 dition, indirectly associated with this remarkable change, 

 which produces greater inconvenience than can be attri- 

 buted to cervical auricles. In many of the situations 

 where canals open on a free surface, the terminal orifice 

 of the canal is, as a rule, surrounded with glands and a 

 collection of tissue, peculiar in structure, termed adenoid. 

 Such glandular collections are more abundant around 

 the terminations of functionless ducts. Some of the 

 more characteristic examples occur in the pharynx 

 marking the inner orifices of the branchial clefts. Of 

 these the most conspicuous is named the tonsil. 



The tonsils are familiar to all as the sub-globular 

 shaped structures lodged in the recesses on each side of 

 the mouth at the spot where the mouth joins the 

 pharynx, or cavity where the nasal and buccal passages 

 become directly continuous. The space between the 

 mouth and pharynx is technically termed the fauces. 

 The tonsils vary considerably in size ; in some persons 

 they are large and prominent, in others small and 

 scarcely recognizable. Structurally they are composed 

 of adenoid tissue, covered with mucous membrane, beset 

 with a number of shallow crypts which secrete thick, 

 tenacious mucus. The niche in which each tonsil is 

 lodged is termed the tonsillar recess, and indicates the 

 exact spot where the second branchial cleft in the 

 embryo communicated with the pharynx ; it is also the 

 spot where the cleft, when persistent, opens internally. 

 The connection of the tonsil and its recess with the 

 second branchial cleft is also indicated anatomically by 

 the glossopharyngeal nerve and the lingual artery, 

 which, in the embryo, are distributed to this cleft. 



