92 ELEMENTS OF APPLIED MICROSCOPY. 
sheep’s wool with camel’s hair will prove instructive, 
showing the lesser length of the latter, its freedom from 
twisting, the scales lying almost flat against the surface, 
and the granular spots in the medulla. Different grades 
of wool may then be studied, contrasting the high-grade . 
merino wools with a diameter of 15 » and 10 or more © 
twists in every centimeter of length with a wool of poor 
quality having perhaps four or five times that diameter 
and not more than one twist in a centimeter. These 
two qualities, fineness and convolution, principally deter- 
mine the quality of wool, although its regularity and the 
projection of its scaly covering are also of importance. 
A sample of wool shoddy may profitably be examined; 
various foreign fibres, dyed fibres, and torn and broken 
fibres will be apparent. 
11. Silk.—One other animal fibre of quite a different 
character remains to be considered, the secretion of the 
silkworm. The larve of many moths spin their cocoons 
out of threads long and strong enough to be used for 
textile purposes, but that of Bombyx mori is cultivated 
most extensively for commercial purposes in China, 
Japan, India, France, and Italy. 
The substance of which silk is composed is poured out 
in a liquid condition from two glands at the anterior end 
of the worm, and when discharged in the air at once sets 
in the form of solid structureless rods. The pair of 
fibres formed by the two large spinnerets are cemented 
together by an incomplete cuticula of somewhat differ- 
ent composition produced by another set of glands and 
known as sericin. Under the microscope such a double 
