usted to the function it is intended to perform. Thus,
to take some further examples, the arteries are admirably adapted as
regards size of lumen, elasticity of wall, direction of branching, to
conduct the blood to all parts of the body with the least possible waste
of the propelling power through frictional resistance. So, too, the
spongy substance of the long bones is arranged in lamellae which take the
direction of the principal stresses and strains which fall upon the
bones in action.

Functional structure may be formed either in the first or in the second
period of development, may be either inherited or acquired, but it
reaches its full differentiation only in the second period, _i.e._,
under the influence of functioning. Practically speaking, functional
structure is directly dependent for its full development and for its
continued conservation upon the exercise of the particular function
which it serves. In the second period, but not in the first, increased
use leads to hypertrophy of the functional structure, disuse to atrophy.

From functional structure is to be distinguished nonfunctional
structure, which has no relation to the bodily functions--is neither
adapted to perform any of these, nor has arisen as a by-product of
functional activity. "To this category belong, for example, among
typical structures, the triangular form of the cross-section of the
tibia, the dolicocephalic or brachycephalic shape of the skull, most of
the external characters distinguishing genera and species, many of the
external features of the embryo which change in the course of
development, besides most of the abnormal forms shown by monstrosities,
tumours, etc." (p. 74, 1910). Non-functional structure is not affected
by functional adaptation, and may accordingly be left out of
consideration here.

Now the influence of functioning upon the form and structure of an organ
is twofold. There is first the immediate change brought about by the
very act of functioning--for example, the shortening and thickening of
skeletal muscles when they act. This is a purely temporary change, for
the organ at once returns to its normal quiescent state as soon as it
ceases to function. Such temporary functional change, brought about in
the moment of functioning, is usually dependent for its initiation upon
some neuro-muscular mechanism, though it may be elicited also by a
chemical stimulus. It is thus always a phenomenon of "behaviour." "From
such tem