uch a
way that the stretching action of the blood-stream brings the vessel to
its proper functional length.

"(6) The power to form, in response to slight increases in longitudinal
tension, new structural parts which take their place alongside the
existing longitudinal fibres.

"(7) The power to regulate the width of the circular musculature
according to the degree of food-consumption by the tissues, in response
to nerve impulses initiated in these tissues.

"(8) The power possessed by the circular musculature of responding to
such continuous functional widening, by the formation of new structural
parts in the circular musculature, and so of widening the vessel
permanently or by this new formation of muscular fibres thickening the
circular musculature.

"(9) The faculty of being stimulated by increased blood-pressure to
produce the same structural changes as mentioned in par. 8, though here
the response is otherwise conditioned" (pp. 126-7, 1910).

It is by virtue of the tissue-properties detailed above that the complex
functional adaptations of the blood-vessels come about.

The development of the vascular system is no mere automatic and
mechanical production of form, apart from and independent of
functioning; it implies a living and co-ordinated activity of the
tissues and organs concerned, a power of active response to foreseen and
unforeseen contingencies. Form is then not something fixed and
congealed--it is the ever-changing manifestation of functional activity.
"Since most of the structure and form of the blood-vessels arises in
direct adaptation to function, the vessels of adult men and animals are
no fixed structures, which, once formed, retain their form and
structural build unchanged throughout life; on the contrary, they
require even for their continued existence the stimulus of functional
activity.... The fully formed blood-vessels are no static structures,
such as they appear to be according to the teaching of normal histology,
and such as they have long been taken to be. Observation and description
of normal development never shows us anything but the visible side of
organic happenings, the _products_ of activity, and leaves us ignorant
of the real processes of form-development and form-conservation, and of
their causes" (p. 125, 1910).

The real thing in organisation is not form but activity. It is in this
return to the Cuvierian or functional attitude to the problems of form
that we hold Roux's