ls of pipe which build a furnace,
or the stones and mortar which make the support of a great railroad
bridge. Yet while the principles of structure are thus simple, for the
general understanding by the student who begins their study the complete
appreciation of the shades of variation, which differentiate one tissue
from another, which define a sound tendon or a ligament from a fibrous
band--the result of disease filling in an old lesion and tying one organ
with another--is as complicated as the nicest jointing of Chinese
woodwork, the building of a furnace for the most difficult chemical
analysis, or the construction of a bridge which will stand for ages and
resist any force or weight.

All tissues are composed of certain fundamental and similar elements
which are governed by the same rules of life, though at first glance
they may appear to be widely different. These are (a) amorphous
substances, (b) fibers, and (c) cells.

(a) Amorphous substances may be in liquid form, as in the fluid of the
blood, which holds a vast amount of salts and nutritive matter in
solution; or they may be in a semiliquid condition, as the plasma which
infiltrates the loose meshes of connective tissue and lubricates the
surface of some membranes; or they may be in the form of a glue or
cement, fastening one structure to another, as a tendon or muscle end to
a bone; or, again, they hold similar elements firmly together, as in
bone, where they form a stiff matrix which becomes impregnated with lime
salts. Amorphous substances, again, form the protoplasm or nutritive
element of cells or the elements of life.

(b) Fibers are formed of elements of organic matter which have only a
passive function. They can be assimilated to little strings, or cords,
tangled one with another like a mass of waste yarn, woven regularly like
a cloth, or bound together like a rope. They are of two kinds--white
connective tissue fibers, only slightly extensible, pliable, and very
strong, and yellow elastic fibers, elastic, curly, ramified, and very
dense. These fibers once created require the constant presence of fluids
around them in order to retain their functional condition, as a piece of
harness leather demands continual oiling to keep its strength, but they
undergo no change or alteration in their form until destroyed by death.

(c) Cells, which may even be regarded as low forms of life, are masses
of protoplasm or amorphous living matter, with a nucleus and fre