lass is exemplified in the _connective_ tissues--tissues
composed of intercellular substance in which at certain intervals cells
lie embedded.

Muscles, nerves, and vessels form a somewhat heterogeneous group. The
idea suggests itself that we have in all three structures to deal with
real tubes filled with more or less movable contents. This view is,
however, inadequate, since we cannot regard the blood as analogous to
the medullary substance of the nerve, or contractile substance of a
muscular fasciculus.

The elements of muscle have generally been regarded as the most simple.
If we examine an ordinary red muscle, we find it to be composed of a
number of cylindrical fibres, marked with transverse and longitudinal
striae. If, now, we add acetic acid, we discover also tolerably large
nuclei with nucleoli. Thus we obtain an appearance like an elongated
cell, and there is a tendency to regard the primitive fasciculus as
having sprung from a single cell. To this view I am much inclined.

Pathological tissues arise from normal tissues; and there is no form of
morbid growth which cannot in its elements be traced back to some model
which had previously maintained an independent existence in the economy.
A classification, also, of pathological growths may be made on exactly
the same plan as that which we have suggested in the case of the normal
tissues.

_Nutrition, Blood, and Lymph. Pus_

Nutritive material is carried to the tissues by the blood; but the
material is accepted by the tissues only in accordance with their
requirements for the moment, and is conveyed to the individual districts
in suitable quantities. The muscular elements of the arteries have the
most important influence upon the quantity of the blood distributed, and
their elastic elements ensure an equable stream; but it is chiefly the
simple homogeneous membrane of the capillaries that influences the
permeation of the fluids. Not all the peculiarities, however, in the
interchange of nutritive material are to be attributed to the capillary
wall, for no doubt there are chemical affinities which enable certain
parts specially to attract certain substances from the blood. We know,
for example, that a number of substances are introduced into the body
which have special affinities for the nerve tissues, and that certain
materials are excreted by certain organs. We are therefore compelled to
consider the individual elements as active agents of the attraction.