96 E VOL UTION AND DISEASE. 



When the inflammation is more intense pus forms, 

 requiring active interference for its relief and cure. 

 There is reason to believe that millers' horses are more 

 liable to acquire these concretions than others. 



There is a third tonsil in the pharynx which deserves 

 some attention, although it is not associated with a 

 branchial cleft. 



When discussing the probable mode of origin of the 

 central nervous system, attention was drawn to a duct 

 which, in the embryo, traverses the floor of a recess in 

 the base of the skull (the pituitary fossa) and opens on 

 the roof of the pharynx. This duct is represented in 

 fig. 25, page 50, and is now regarded as a remnant of 

 the ancestral vertebrate gullet. The pharyngeal orifice 

 of this duct is surrounded by a collection of tissue 

 which, structurally, is identical with the tonsils, and the 

 organ has been named in consequence the pharyngeal 

 tonsil of Luschka, in honour of its discoverer. As far 

 as is at present known this organ has no function, but it 

 is often a source of trouble and inconvenience, mainly 

 in children, inasmuch as it is especially prone to enlarge 

 and obstruct the eustachian tube, producing deafness. 

 It also interferes, when large, with the free passage of 

 air through the nostrils, to such a degree as to require 

 surgical interference. 



As most of the vestigial structures considered in this 

 chapter are the outcome of modifications induced by 

 the change from an aquatic to a terrestrial mode of life, 

 we may conclude .it by briefly considering the fibula in 

 relation to Pott's fracture. No bone of the lower limb 

 of man, excepting the neck of the femur, is so liable to 



