256 EVOLUTION AND DISEASE. 
deposits constitute the most constant pathological con- 
dition in gout. 
There is a parasitic affection common in swine 
(and not infrequently found in man), to which atten- 
tion has of late years been largely directed, known — 
as trichinosis. The parasite TZvichina spiralis when 
hatched finds its way into the voluntary muscles of — 
man and the pig, there becomes encysted, and in due 
course is surrounded by calcareous particles. The 
encysted worms are visible to the naked eye in cut 
sections of muscle as small dots. In 1866 Virchow © 
detected in a piece of ham some small white concretions 
which were regarded as trichinze; but on examination 
were found to be of crystalline structure, and to furnish 
the reaction for guanin, a crystalline nitrogenous body 
resulting from chemical changes in animal tissues, first 
discovered by Unger in Peruvian guano. 
Guanin seems to be very widely distributed in the 
animal kingdom ; it occurs in fish, the excrement of 
spiders, in the pond mussel, the pancreas and liver of 
the horse, and the skins of frogs and lizards. It responds 
to easily applied chemical tests. The interest of guanin 
for us centres itself in the fact that it produces lesions 
identical in their pathological anatomy with gouty 
lesions, that is to say, it becomes deposited in cartilage 
and fibrous tissue, forming deposits exactly resembling 
the urate of soda deposits in man. Such resemblance is 
not confined to naked-eye characters, but extends also 
to the microscopic details as is shown in the drawing 
(fig. 124), taken from Dr. Mendelson’s admirable contri- 
bution to this interesting subject. When examined in 
