restored by ingrowth from the margins, and there is evidence that some
of the secreting glands may be reproduced.

Hollow viscera, like the œsophagus and urinary bladder, in so far
as they are not covered by peritoneum, heal less rapidly.

_Nerve Tissues._--There is no trustworthy evidence that regeneration of
the tissues of the brain or spinal cord in man ever takes place. Any
loss of substance is replaced by cicatricial tissue.

The repair of _Bone_, _Blood Vessels_, and _Peripheral Nerves_ is more
conveniently considered in the chapters dealing with these structures.

#Rate of Healing.#--While the rate at which wounds heal is remarkably
constant there are certain factors that influence it in one direction or
the other. Healing is more rapid when the edges are in contact, when
there is a minimum amount of blood-clot between them, when the patient
is in normal health and the vitality of the tissues has not been
impaired. Wounds heal slightly more quickly in the young than in the
old, although the difference is so small that it can only be
demonstrated by the most careful observations.

Certain tissues take longer to heal than others: for example, a fracture
of one of the larger long bones takes about six weeks to unite, and
divided nerve trunks take much longer--about a year.

Wounds of certain parts of the body heal more quickly than others: those
of the scalp, face, and neck, for example, heal more quickly than those
over the buttock or sacrum, probably because of their greater
vascularity.