96 EVOLUTION 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 skul]l (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 
alas ye fs 
