ls are subject to the most rapid changes of
material? 444. May animals be situated so that they require no
nutriment? What is related of frogs?

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445. The renovation of the bone, muscle, ligament, tendon, cartilage,
fat, nerve, hair, &c., is not perfected merely by the general
circulation of the fluid which is expelled from the left side of the
heart, but through the agency of a system of minute vessels, which,
under ordinary circumstances, cannot be seen by the eye, even when
aided by the microscope; still, minute as they are, the function of
these agents is necessary to the continuance of life. They are the
smallest capillary vessels.

446. "As the blood goes the round of the circulation, the nutrient
capillary vessels select and secrete those parts which are similar to
the nature of the structure, and the other portions pass on; so that
every tissue imbibes and converts to its own use the very principles
which it requires for its growth; or, in other words, as the vital
current approaches each organ, the particles appropriate to it feel
its attractive force,--obey it,--quit the stream,--mingle with the
substance of its tissue,--and are changed into its own true and proper
nature."

447. Thus, if a bone is broken, a muscle or a nerve wounded, and, if
the system is in a proper state of health, the vital economy
immediately sets about healing the rupture. The blood, which flows
from the wounded vessels, coagulates in the incision, for the double
purpose of stanching the wound, and of forming a matrix for the
regeneration of the parts. Very soon, minute vessels shoot out from
the living parts into the coagulum of the blood, and immediately
commence their operations, and deposit bony matter, where it is
required to unite fractured bones, and nervous substance to heal the
wounded nerve, &c.

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445. Show how the renovation of the bones, muscles, &c., is perfected.
446. What is said of the office of the nutrient capillary vessels?
447. When a bone is fractured, by what process is it healed?

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448. But the vital economy seems not to possess the power of
reproducing the muscles and true skin, and therefore, when these parts
are wounded, the rupture is repaired by a gelatinous substance, which
gradually becomes hard, and sometimes assumes something of a fibrous
appearance. It so perfectly unites the divided muscle, however, as to
restore