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AREOLAR TISSUE, TENDONS, AND MUSCLES. 201

divide and form a complete network of capillaries. This network,
when present, lies below the basement-membrane, projecting into the
papillae of the papillary layer. The tubular and other glandular
apparatus are abundantly supplied with nourishment by this vascular
rete, which surrounds them for that purpose.
Lymphatics are found in the form of a network in the mucous mem-
brane, communicating with larger vessels in the submucous layer.
Nerves.—When muscular fibres exist in mucous membrane, the nerves
are chiefly distributed to them, also to the glandular aj)paratus ; there
are also ganglionic plexuses in the submucous tissue. Some terminal
nerves have been found to pass through into the ejjithelium and ter-
minate between the epithelial cells : this appears to have been demon-
strated in the epithelium of the mucous membrane of the palate of a
rabbit.^
Pupiike and Villi are found upon some parts of the nuicous mem-
brane : the former are conspicuous upon the tongue, as hereafter described,
and the latter are abundant and fully developed on the mucous membrane
of the small intestines.
The Secretoky Glands of the Mucous Me^mbeane.—The
secretory glands are organs which vary in structure and in their secre-
tion part of them are situated within the mucous membrane, \\hile
;
others are at various distances from it, though in all instances their
ducts open upon its surfiice. Although they differ considerably in their
function and locality, they have an embryonal derivation similar to that
of the epithelial tissue upon which their ducts empty their fluid ; their
development usually begins during intra-uterine life. These glands are
essentially made up of one or more layers of secreting cells, usually
resting upon a basement-membrane. Immediately below the basement-
membrane there is an abundant supply of fine blood-vessels ; when the
membrane is absent, the vessels are in close contact A\'ith the attached
ends of the cells.
Enlargement of secreting surfaces of any part is generally by reces-
sion or inversion (there are examples in nature Avhere the increase of sur-
face is produced by protrusion or an elevation), which is carried from
very simple forms to various degrees of complexity. The first or most
rudimentary of these varieties is a recess, the result of the dipping down
of the jNIalpighian layer into the subepithelial tissue, and forming what
is sometimes called an epithelial sac, the shape of which is tubular or
saccular. This simple tube sometimes lengthens considerably and coils
upon itself, forming a ball, which is known as a coiled tube. The sweat-
glands of the skin are of this kind, and are formed by the dipj)ing down
of the lower strata of the cells or the embryonal Malpighian strata.
These embryonal cells are not columnar in shape, as found in adult
tissue, and have been described by Zeigler^ thus: "The cell by itself
appears originally as a microscopic mass of pale, finely-granular matter,
the so-called protoplasm. It usually contains within it a nucleus—that
is to say, a structure like a tiny vesicle—whose form may be round,
^ Quain's Avnfnmy.
2 From a paper read by Dr. W. X. Suddutli before the Odontographic Societv,
Oct., 18S4.